Light is, like most lay parlance, an inadequate human characterization of the underlying phenomenon. It isn't necessary to know how light works in order to appreciate it or use it, and so most people who are not physicists don't bother to learn, the same way that only a tiny percentage of earth's population really understands the internet. It's somewhat immaterial. Monkeys don't need to define gravity in order to fall down, after all. Light is - and for most people, that's good enough. That is to say, to quote the poet T.S. Eliot: "Light, light, the visible reminder of invisible light."
It is exceedingly relevant, however, when one is a being made of light.
Castiel's true form is made of electromagnetic radiation. He has spent the majority of his life, if you really want to add it all up and average the whole thing out, as a wavelength of celestial intent.
The thing about being made of light: it's light in the physics sense of the word. Castiel's waves are gamma, x-ray, micro, and radio. He's visible light too, of course, a visible light so intense that it is blinding to most humans.
The light takes on a form, sometimes. Mostly, Castiel lets his true form exist in propagation and frequency, as wave and particle, as a quantum being. He was created in an image though. He can become something with a shape, it's just that the scope and boundaries of that shape are as practically indescribable in human language as the shape of gamma rays. His true form runs along the full spectrum of light, visible and invisible, a constant movement that would be dizzying to the human eye if it were even conceivable. He hums and thrums and glows and flashes with celestial incandescence. If there is an outline to him, it might have almost the vague shape of man. He has limbs and a center, anyway. The wings are the most probable thing about him, always tucked away on earth as invisible light, radiation that sometimes makes the electricity in a room blow out. Nevermind the multiple faces or the number of those limbs. Nevermind the fingers, toes, claws, and hoofs that God saw fit to give his first creations, when he was trying out the materials of the universe. At least humans seem to understand the wings.
Dean used to call the angels "junkless" as a harmless insult - a quip, Cas knows now, to mask his initial fear and awe of them. He's not exactly wrong or exactly right.
Gender is another concept humans like to use a word for without really understanding what it means. Like light, it is a spectrum. Like light, some gender is visible to the naked eye and some of it is not.
Cas didn't really understand human gender until he fell. He and his siblings call each other "brother" and "sister", but there are no direct translations in Enochian. The word for sibling is something like brethren , although even that is not quite right. It means something closer to companion at arms, something like blood of our father or perhaps weapon of our father . Father, too, is not wholly accurate. It is father and mother, both creator and commander. Family is just a term for a unit of war.
Humans, though, have so many gendered words. In some languages, the word itself may have an explicit gender but the object carries little or no connotation. In others, like English, many words have a hidden gender, invisible, something you have to have a deep cultural knowledge of to understand.
Castiel for the life of him can't understand why he shouldn't call a man "pretty," for instance. Or why Dean insists on "she/her" pronouns for his car. Or why to penetrate in sexual intercourse is "masculine" and to be penetrated is "feminine". Or why and how clothes are part of performing gender.
"It wasn't always this way," Cas tries to explain to Dean one afternoon at a thriftshop when Dean makes a disgusted noise and a rude comment about a pink shirt in the men's section where he's rooting around for his usual flannel.
"I mean, for one thing, the feminization of pink was propagated almost entirely by Nazi Germany. Before the Nazis began using a pink triangle to mark homosexual detainees, there was no rigid gender assignment to color. It fluctuated over the years, and at one time pink was even considered the more masculine color because it was more "vibrant". It wasn't until after World War II that it became cemented as something that men shouldn't associate with. So really, the gendering of color is steeped in historic homophobia."
Dean, at this point, pinches the bridge of his nose and says "Cas," in an exasperated way.
"But it's also, if you want to take the broader view," Cas goes on, rifling through the hangers himself. "That you didn't always have this dichotomy of gender. I mean, how do certain pants make you one gender or another? What if you went nude, would that be neutral?"
Dean makes a little choking noise on the other side of the aisle.
"I suppose you'll say that genitalia would be a clear indicator of gender, but like I said, historically speaking, this binary of yours has a relatively short life so far. There's very little that reproductive organs tell me about what, say, a person in Namibia in 200 BC would identify as. Or what the word for that identity would mean about what you'd call gender roles."
"Dude, I get it, I'm on board. I'm sorry about what I said about the shirt, okay? Please stop talking about this in the Goodwill."
Cas certainly didn't have a gender before he fell. Or, if he did, he had a gender like the Spanish word for "homework" has a gender - something purely linguistic. Or, maybe, it was purely theoretical - he had a gender in the way that light is both a particle and a wave. Or, perhaps, it was just that Cas had never thought much about it as a being of light perceived primarily by other beings of light.
Cas had other vessels before Jimmy Novak. Not many, in the grand scheme of things, but a few. Often enough to have had a diverse experience of human bodies.
If he's being honest, he never really noticed the gender or sex of his vessels. He'd barely been aware of their reproductive organs or chromosomes or hormones. It hadn't made the least bit of difference. All that mattered was the bloodline and compatibility.
It's been different, with this one.
Cas has been brought back from the dead so many times, has been remade, re-shaped, re-purposed, that he's pretty sure… Well, he's pretty sure whatever was left of Jimmy is gone from this body. He's pretty sure that, somehow, now that he's fallen, even though he still has his grace (sometimes), this body has become his. That even though he wasn't born to earth like Anna was, this is just his form on earth now, because it's the form he prefers. He can barely imagine what it would be like to take another vessel, even though he could, even though maybe, if he picked a different kind of body, he could have the things he wants.
Cas didn't really have, or at least know, a gender until Jimmy. And it wasn't his vessel who gave him one, not really.
Cas doesn't engage much with pop culture. He's never had the leisure. The only media he ever kept up with from Heaven were seminal texts on God or religion. He's read the Hebrew Bible, of course, and the Christian Bible with the apocrypha, the Quran, the Talmud, the texts on Kabbalah, the writings of all denominations of prophets, the texts from Joseph Smith's tablets, the Vegas, the Tao Te Ching, the Book of Shadows, and many others. He has also read a number of books banned in the name of God - all of them for frivolous or veiled political reasons. He's read Judy Blume's work, which he liked, and Steinbeck's, which he didn't. He skimmed Harry Potter . He's enjoyed many of the censored LGBTQ+ novels: Annie on My Mind , Aristotle and Dante Discover the Secrets of the Universe , A Single Man .
Years ago, a couple of decades, maybe, he'd picked up another children's series supposedly promoting the devil. This one had been a little different than the others. Usually, Cas found all the censorship and talk of heresy to be nothing more than projection and hypocrisy. His Dark Materials , on the other hand, was a scathing rebuke of God and religion.
Cas wasn't offended by it. Humans could believe or not believe. It wasn't his job to convince every single human being of the divine justice of The Plan. So Cas didn't care about the thinly veiled religious commentary, wasn't bothered by the gay angels or the literal war against God. He was a little surprised to find some merit in the accusations of heresy, but he'd tucked it away in the back of his brain and hadn't thought much about it.
In the world of these books, humans have a physical manifestation of their soul that appears at their side as an animal, called a dæmon. Children's dæmons change shape until they are older and settle into whatever form they will take for the rest of their lives. For the two protagonists of the novels, their souls settle when they first lay a hand on one another's dæmons. They couldn't bear to change shape after being touched by the person they love.
Cas thinks of these books for the first time in years when he's brought back to life after Raphael blew him to pieces all across Chuck's living room. He finds himself dressed as Jimmy Novak again and he's not sure why, because already he's pretty sure that Jimmy is gone. But he's relieved, grateful, because he's become attached.
At first he thinks he's just being sentimental, but then he sees Dean's face when Dean realizes Cas is alive. The recognition and affection in Dean's eyes is all it takes. This is how Dean sees Castiel: a man in a trench coat with perpetually windswept hair and rough stubble. That's who Dean sees and trusts and who he has, sometimes, looked at with love in his eyes.
Cas never needed to be seen before he fell, either.
This body, this form, is the one that Dean has looked at, has touched, has hugged, has called family.
Cas finds he can't bear the idea of being somebody else.
A week or two after Cas tried to explain about the complicated and mostly colonized idea of gender, Dean comes out to the kitchen one morning in a gray and pink flannel shirt. Sam is sitting at the breakfast nook in his t-shirt and sweatpants, drinking a green smoothie and flipping through newspapers. He looks up at Dean's entrance and frowns, forehead creasing as he takes in his brother.
Dean pours himself a cup of black coffee and ignores the glass of smoothie Sam set aside for him. He sits down across from Sam, all purposeful nonchalance, and only then glances at his brother's face.
"You leave your mouth open like that, Sammy, the flies'll get in," Dean says, taking a sip of his coffee. "You got something to say?"
"Uh, no," Sam says. "No. I just. Uh. I like your shirt."
Dean glares at him. He's rather deliberately not looking at Cas at the end of the table.
"Glad to hear it."
"I like it, I'm just surprised you do."
Dean takes a leisurely sip of his coffee before answering. "What's the matter with it?"
"Nothing." Sam seems to be restraining himself for a moment before he breaks. "It's just, you know, one time we had to wear "salmon" colored shirts to blend in with a company uniform working a job, and afterwards you burned them both because you didn't even want to look at the color on me. And one time in Ohio, that girl gave me a pink tie for Christmas, remember? And you said -"
"Okay, okay, I get it. Look, you a Nazi, Sam?"
"What?!" Sam blinks, confused and halfway to offended. "What kind of question is that? Of course not."
"Good, well me either. You and I may have done some morally gray things in our past, but I got one rule to figuring out what's right and wrong in this life: if you look around and realize that you are standing on the same side of a line as actual and literal Nazis, you're probably on the wrong side."
"Dean, I have no idea what you're talking about right now."
Dean sighs and glances briefly at Cas. "Apparently we only think of pink as a feminine color because it was associated with gay prisoners in the internment camps in World War II, and, y'know, homophobia took it from there. So. I'm giving up my crusade on the color, okay? Just let me be a better person, huh?"
Sam laughs and shakes his head, shaggy hair flopping half into his tired face.
"Okay, sure. Yeah. Although, you know, it's kinda funny, drawing the line at Nazis. You know we've worked with actual and literal demons, right?"
Dean scowls and flicks his untouched smoothie at his brother.
Cas, for his part, just smiles into his own cup of coffee. He can see how tense Dean is, how even just this tiny act of rebellion against the norms he's been taught, against what is probably John Winchester's voice in his head, is making him even more defensive than usual. The next day, Cas sneaks into Dean's room while he's out and steals the shirt. It still smells like Dean. When he wears it around the bunker a few days later, Dean notices and smiles, and Cas thinks Dean understands why he took it, just like Cas understands why Dean wore it in the first place. They're having a conversation without speaking. Showing instead of telling.
Dean is the only person Cas has ever really been able to do this with.
It might be easier, Cas sometimes thinks, if he didn't care about the way his vessel appears now. If he hadn't started to think of himself with human pronouns, hadn't yearned with this body and found that he liked it in a way that feels - and Cas can't begin to explain what this even means - male.
It isn't so much the gender norms he identifies with. He doesn't really understand these well enough for them to factor into his identity. And it isn't just being perceived by Dean, or by the world at large. That's a part of it, of course, and it's how he first realized an important part of what he was feeling, but…
Honestly, Cas only understands gender in the way he understands love - it's a feeling you don't know until you do.
The irony in this is not lost on him. To realize that he likes being a man at more or less the same moment he realizes he's in love with one.
Castiel is utterly indifferent to sexual orientation. But Dean isn't.
It would be easier, maybe, if Cas could bring himself to find a vessel that Dean could fall in love with. He thinks there's a chance, maybe, that if they had the relationship they have, if they kept that love and that tension, between Dean and a female vessel…
But he can't.
Castiel is a being made of light, but he's this, too. This is just the human manifestation of his soul.
A few months after the shirt (Cas keeps it, he just doesn't wear flannel much), Dean has one of his screaming nightmares. The last time Sam tried to wake him up, Dean nearly shot him (Sam ducked), and Dean made him swear not to try again.
Cas is still an angel, though, and mostly bulletproof.
Dean is still tossing and turning when Cas comes into his bedroom and closes the door gently behind him. The dark doesn't bother him. He can still make out Dean writhing, screaming hoarsely, with his arms pinned to the bed like they're tied there, his torso bucking away from something or someone.
"Dean," Cas tries. Then louder. "Dean."
Dean whimpers and mumbles "No. No, no, please, no."
Cas approaches the bed.
"Oh God, no, please. Sam. Sammy. Please ."
"Dean," Cas says again, and puts a hand on his shoulder.
Dean bolts upright, gasping, one hand going up to claw at his own chest, the other grabbing Cas' hand in a vice-like grip.
"Dean, it's me. It's okay."
Cas wonders belatedly if he should have turned on the light. He can see in the dark, but Dean can't.
Dean's quick breathing continues for a beat and his nails dig deep into the skin of Cas' hand. Then suddenly he lets go. His shoulders slump and his breath turns into something that sounds suspiciously like a sob.
"I'm up, Cas." Dean's voice sounds defeated.
"Are you… okay?" Cas winces as the words leave his mouth. It's the human thing to say, but he knows the answer.
"Yeah, sunshine, I'm good."
Cas doesn't really understand why Dean calls him this sometimes. He knows it's a term of endearment, and he knows that Dean says it sarcastically, but it's also… it's only ever with Cas.
"Dean," Cas says again. He waits, hovering near Dean's bed in the dark. Dean has hunched in on himself, knees drawn up to his chest.
"Want to talk about it? No. Thanks for shutting me up. Know it tears up Sam."
Cas wants to say that Sam isn't the only one, but he doesn't. He waits a moment longer, feeling awkward and useless, then takes a step back.
Dean's hand shoots out and grabs Cas' wrist. Cas gets the impression that the movement was involuntary.
"Wait." Dean is silent for a full minute. Cas let's him struggle to the words. He doesn't know how else to help him but to let him help himself.
"Stay?" Dean finally gets out. It's a whisper and more of a plea than Cas was expecting.
"Of course, Dean. I'll watch over you."
Dean doesn't let go of his wrist. He tugs a little, and Cas lets himself get pulled right up to the bed, but then he stops, because…
"Cas," Dean says. "Come on. Please."
Cas wants to tell him that all he had to do was ask, all Dean's ever needed to do was ask and Cas would be there. He doesn't say anything, just sinks onto the bed next to Dean's hunched form.
Dean lets go of his wrist and leans his forehead against Cas' shoulder instead. Dean shudders, a whole body movement that shakes the bed. Cas hesitantly touches Dean's hair, and when he doesn't stop him, Cas runs his fingers through it. He's always wanted to do that.
Dean makes a noise that is something between a sigh and a sob and his forehead presses harder into Cas' shoulder, like he's trying to burrow closer to him. So Cas, not at all sure if this is what Dean is asking for, puts his arm around him.
For a second, Dean tenses. He stays frozen where he is, breathing labored. Then he seems to let go, and he collapses into Cas, face sliding into Cas' collarbone, wrapping his own arm around Cas' waist.
Cas sits there for what feels like a long time, holding Dean tight with both arms now, rubbing up and down his spine with one hand. Dean doesn't cry. He just breathes heavy and sometimes brokenly into Cas' shirt.
"Hey Cas?" Dean whispers when his breathing has finally evened out and whatever this is that they are doing has started to feel an awful lot like cuddling.
"I'm sorry about the stupid pink shirt thing."
Cas lets out a surprised huff of laughter. He chances sliding his fingers back onto Dean's hair, and Dean doesn't protest.
"Why are you thinking about that now? It doesn't matter."
Dean shrugs against him. "I don't know. I feel like you were trying to tell me something important and I just waved you off."
"I don't care if you dislike the color pink, Dean."
"Yeah, but I…" Dean trails off, sighs, and sits up. He dislodges Cas' hands, and Cas fights the urge to reposition them, to reach for Dean.
Dean's looking at him, and Cas doesn't know how much Dean can see in the dark, but it seems like he's searching Cas' face.
"I know I've said some things and done some things that might make you think that I… I don't know, that I'm… bigoted or whatever. Narrow-minded. And yeah, I'm kind of… I mean, I like old cars and old music and old movies, but I'm not that old-fashioned. You know, besides you, Charlie's probably the best friend I've ever had. So, I'm just saying, I might not understand everything right away, but doesn't mean I won't accept it."
Dean stops. Cas tries to piece together what he's saying, but he's drawing a total blank. Dean is still near enough to smell - gunpowder, whiskey, and leather seats. Dean always smells like the job.
"Okay," Cas says finally, because Dean is clearly waiting for some kind of response.
Dean's sighs and runs a hand across his face. "Cas. I'm just trying to say, you can tell me stuff, if you want to."
Cas isn't sure how loaded of a statement this is supposed to be. Can he tell Dean that his gunpowder-whiskey-leather smell is the first thing Cas associates with home? Can Cas tell him that sometimes when they're driving at night, Cas looks at the stars and his chest aches so deeply for Heaven - not as it is, but what he once believed it to be - that he thinks he might die? Can Cas tell him that when Sam and Dean are both asleep in the bunker, sometimes Cas lays down on the cold hard floor and closes his eyes and imagines what it is like to dream?
"Uh," Cas stalls. And then it finally comes tumbling into place, and he laughs as much from relief as anything else. "Oh. Oh! You're trying to tell me I could come out to you, aren't you?"
"I mean, not to force your hand, but… yeah. Kinda."
"Come out as what, though? What did you think I was trying to say?"
Dean shifts on the bed, the mattress creaking slightly. "I don't know. Non-binary, maybe? Or… agender? Christ, Cas, I don't know what the words are. I just thought maybe that's what you were trying to say and I blew you off."
Cas laughs again. He doesn't know how to explain it to Dean, but he tries. "I appreciate what you're trying to say, Dean, but you've got it backwards. Angels, we aren't typically given a gender at birth and our true forms don't correspond to your human categorization of sex, so it's not… Most of us stay that way, if we develop enough of a sense of self to have an identity at all. I… I suppose that I was assigned more of a non-binary identity at birth, but… I do identify as male. Ish."
"Oh," Dean says. "Okay. Cool. Thanks for telling me."
Cas feels a rush of warm affection for Dean's awkwardness. His genuine attempt at caring.
"For what it's worth," Cas says, before he can stop himself. "I'm probably gay."
Dean is quiet for a moment and Cas closes his eyes.
"Oh," Dean says. It's softer this time.
"Yeah," Cas says, wishing already that he could take it back. The whole night has been strange.
"Um," Dean says. "Probably, huh?"
"Well, it's pretty theoretical at the moment."
"Theoretical," Dean echoes. And then suddenly it's Dean's fingers in Cas' hair and Cas opens his eyes with a start.
"I'm no scientist, Cas, but…" Dean doesn't finish the sentence. Or he does, but by pressing his mouth to Cas' lips.
In compliance with both the most recent and not so recent etymology, Cas is pretty definitely gay - he's a man(ish) in love with a man, and he's happy.
Light, it turns out, is a perfectly adequate conduit for love.