It came in 3 unassuming moments in his life. If asked, Pro hero Ground Zero will deny the possibility of a guardian angel until his dying breath, but the 3 letters he so carefully stored away in a shoebox inside his cabinet beg to differ.
The first letter he received was way back in middle school. It came the morning their homeroom teacher was due to discuss their selection for high school choices. Katsuki woke up as usual, had breakfast with his dad and his usual screaming match with his mom. After his bath, he went upstairs to prepare for the day. There, sitting unassuming in his desk, was a letter addressed to him. A girl did give him a letter yesterday. Probably another confession letter, he thought smugly, but then he remembered that he exploded that thing yesterday, causing the girl to cry and run away from him. Curiosity now piqued, he opens the letter and on it were written the words "Don't be an ass. Don't let them tell him that he can't make it."
"The fuck!?" was Katsuki's first thought, but he was quickly taken out of it when his mom yelled for him to hurry up, otherwise he will be late. Tossing the letter inside his drawer, he quickly gets himself ready for school. "That shit can wait" he mutters to himself.
In school and to no one's surprise, Izuku raises his hand and says he plans on applying to UA as well. Immediately, his classmates start snickering and makes fun of the "quirkless wonder." Katsuki used to bully Izuku in the past, and still does on occasion, but him taunting Izuku has since lost it's appeal. He doesn't start them anymore, but when prompted, he always gives out a little insult here and there.
Izuku stammers and nearly shakes, but he doesn't back down even when his homeroom teacher tells him to select a more "realistic" school of choice. It was then that Katsuki remembered the mysterious letter he received that morning. "Don't be an ass" the words kept repeating itself in his head.
-- "atsuki, what do you think!?" One of his classmates bumps his shoulder, likely trying to get him to make fun of Izuku. Izuku, who was crying silently but hasn't made a move to concede his dream of going to U.A. Izuku, who used to be his childhood bestfriend. Izuku, who was since diagnosed quirkless and has since been the subject of ridicule at his school.
In a strange mix of pity and reluctant awe, Katsuki opened his mouth and said "Tsk, leave the loser alone. If he wants to get killed that badly, then let him apply for U.A. As if he'll get in anyway." Izuku looks at him slackjawed, but as an impromptu leader within the classroom, all of his classmates decide to drop the subject
After school, Katsuki goes straight home and takes a nap. When dinner time came, his mom casually mentions that she and her friend, Midoriya Inko will meet tomorrow night for some spa date. Katsuki just dismisses his mom's rant, but then Mitsuki casually says " Oi brat, Izu-chan called too, says he wants to say thank you. I didn't wake you up cause I know you'll be pissy. The hell did you do?"
"I didn't do shit, old hag! He said thank you to me, why are you assuming I did something wrong!?"
"Because you're you" is his mom's retort. After dinner, Katsuki locks himself in his room and thought back on Izuku's phone call, getting a weird sense of accomplishment all the while.
The second letter came during his 2nd year as a high school student. Katsuki and Izuku both got into U.A. It was a surprise to everyone when Izuku somehow passed the entrance exam. Still quirkless, Izuku did poorly on the physical part of the exam, but aced the written part, barely scraping enough points to get into the hero support courses. Katsuki and Izuku have since developed a tentative sort of understanding regarding their "friendship." If asked, yes they knew each other, but other than that, they never sought each other out. It was a weird thing, being apathetic to the existance of your once childhood bestfriend, but Katsuki and Izuku make it work. Even though Katsuki hears Izuku's name in passing, he never seeks out the nerd.
At U.A., Izuku's penchant for anything hero-related was considered endearing and not a total nerd-alert, so much so that Izuku has grown comfortable in his own skin. Before long, he started having friends of his own. Izuku has long since accepted the fact that he has no quirk, but still wants to do work in the pro hero circuit, so he decided to enroll in the hero support program at U.A.
During their second year, Katsuki's homeroom teacher mentioned that in preparation for their future pro hero careers, they need to get acquinted with other works related to their chosen field, which means that tomorrow, they will be working with the hero support students to work on possible support items they can use.
Katsuki trudges to his room at Heights Alliance and decides to do his homework before hitting the gym. There, stuck between his math and hero history notebooks, was a letter with his name on it. Immediately remembering the note he got in middle school, Katsuki hurriedly opens the letter and reads it. It was a lot longer than the first, with notes on always being careful and not being complacent about his own goals. What caught his attention though, was the footnote. Written in red ink were the words "it's okay to be confused at times. You're still young, so you got time to figure it out."
Katsuki hurriedly calls his homeroom teacher and asks permission to leave the dorm and go home for the weekend. Aizawa doesn't see any harm in it, and quickly agrees (if only to get Katsuki off the phone).
Morning comes and Katsuki is bored out of his skull. When Aizawa-sensei faced the class and started introducing the support course students, Katsuki was casually listening but felt himself bummed when Izuku wasn't with the class. Perhaps he was doing this shit with class 2-B, Katsuki wasn't sure. "Well, I can just make fun of Deku's scrawny little ass next time", Katsuki thinks. Aizawa then starts assigning their class with their support course partners, when a mop of hair pops inside their classroom and asks if this is the 2-A class. Katsuki looks up just in time to see Aizawa use his capture weapon to shush the mumbling support course student who was spouting excuses as to why he was late. Aizawa then releases his capture weapon and loudly exclaims "and just for that, you will be assigned to my problem child. Row 4 aisle 3, you'll be working with Bakugou Katsuki."
This is it, Katsuki thought to himself. He's screwed. As Aizawa lowers his capture weapon, Katsuki is immediately assaulted by Izuku's blushing face, likely embarrased from being called out by a pro hero for his clumsiness. What he was not prepared for, was the sight of his childhood friend. No longer a scrawny kid, Izuku has hit a growth spurt and has since learned to do basic toning exercises by the looks of it. Somehow, he's also developed a sense of style cause his hair is no longer an unruly mess of green hair, but it was shaved a bit at the edges and is parted, accentuating his dimples and the freckles on his cheeks.
Izuku hesitantly walks up to him and gives him a tentative smile. "H..hi Kacchan" Izuku stammers. Katsuki was not prepared for Izuku's smile. Not wanting to show his internal freakout, Katsuki says "you got tall, but I'm still taller, nerd" and then Izuku full on laughs at him. It was then that Katsuki realized that Izuku's laugh maybe has a nice ring to it. It was literal years since he heard that laugh, and he was not assaulted with good childhood memories. Nope, not at all.
The thing was, Katsuki has since come to terms with his sexuality. Sure, he dated men and women before, but he somehow leaned more on the masculine side when choosing his dates, and he is starting to realize that his once scrawny childhood bestfriend's smile makes him feel prickly but in a good kind of way.
Katsuki was brought out of his musings when Aizawa began to drone off on what they should do for the next hour of class before zipping himself up in his sleeping bag. Izuku then picks up Aizawa's queue and starts asking Katsuki questions about his quirk. It was then that Katsuki started to notice the little things, like how Izuku's drawings and note taking have gotten more meticulous over time. How his line of questioning about the intricacies of his quirk were more profound. No longer was Izuku always rambling about how cool and powerful his quirk is, he's now able to spout information on how he thinks Katsuki's present hero costume can be altered, so that he can get rid of his bulky gauntlets while still maintaining the same level of firepower he always had. Katsuki was left with nothing, but to answer the questions thrown his way. Before he knew it, the one hour mark was up, and Aizawa-sensei was emerging from his sleeping bag on the floor.
"Uhm, thanks for not exploding me there Kacchan" Izuku rambles. Wide-eyed at how the (not) nerd looks, Katsuki just grunts and says "whatever." Izuku takes it as some sort of sign and just smiles and says "So I'll need your number so we can meet up next week." That got Katsuki's mind reeling and immediately tries to think of reasons why Izuku of all people will ask for his number. He's taken out of his internal monologue when Izuku says "Oh wait, it's probably better if I email you using your school email handle, atleast I can send you progress reports on your costume upgrades. Well, I'll talk to you next week. See ya!"
With that, Izuku and his entire class all bow their heads and Aizawa sheperds them outside the classroom, probably to talk to them for a bit.
After class ends, Katsuki decides to forgo his training and holes up in his dorm room, just staring at the anonymous letter in his grasp. He's kinda screwed. He knows, but he grins despite himself. He's looking forward to the weekend, when he can keep the second letter he received together with the first.
The third letter came that morning. Fast forward 7 years, with lots of problems, misunderstandings, and a whole lot of crying (on Izuku's part, Katsuki swears), he and Izuku have managed to repair their ruined friendship and has since learned to exist together in the same room and even work together on occasion. Katsuki graduated as one of the Big 3 at U.A. and Izuku was scouted by a support company during his third year internship, and has since been working with a reputable support item company right after graduation.
Katsuki has come to terms with the idea that yes, his longing for his childhood bestfriend was not as platonic as he used to think (I'm just making up for lost time), but rather because he may or may not have developed romantic feelings for the (not) nerd. After finding out from class 3-A that Katsuki was bi, Izuku has been smiling at him more, and has even invited him out several times, always in the guise of "Kacchan, I want to discuss some possible costume upgrades, are you free?" And if they just so happen to meet up at some chic coffee shop or a cute cake shop, then it is what it is.
7 years into their tentative "friendship", Katsuki was hit with the realization that he really likes Izuku, and wants to ask him to be his boyfriend. So last week, he casually invited Izuku to their favorite izakaya that's situated in between his hero agency and Izuku's company. After arriving at his apartment to prepare for his "not date", he notices a letter in his mailbox. Seeing the same envelope and lettering, he quickly runs inside his room and opens his cabinet. Underneath his spare boots, he takes out the shoebox containing the first 2 letters he got. All the same envelope, same handwriting, and Katsuki has yet to figure out who sends these to him. Not wanting to jinx it, Katsuki sits on his bed and opens the new letter. The only words written inside was today's date, the restaurant he was supposed to meet Izuku in, and the words "Go to the other place" in bold black letters. Katsuki snickers and calls Izuku and tells him to meet him at that restaurant that serves katsudon instead.
Later that night, while Izuku snuggles with his boyfriend on his couch, Izuku jumps up and says "Oh yeah Kacchan, I forgot to tell you, but that izakaya we were supposed to go to got attacked by a villain earlier, can you believe that!? Good thing Red Riot and Uravity were on patrol so they got everything under control. Hey, can we drop by the izakaya tomorrow? I wanna check things out, just wanna make sure that obasan is okay." Katsuki just snuggles into Izuku more. "Sure nerd, whatever you want."