That mask isn't like the others. It just isn't. The others are like any other mask, besides the magical powers of course. They can hide his face, but they can't hide his heart. Link has never felt like a Deku Scrub or a Zora. Not really. The souls that helped create his masks are long gone, leaving only him. The masks only work to change his outward appearance. He can look like those spirits without really becoming them.
That's not how it feels when he's wearing the Fierce Deity mask, though. When he wears that mask, it's like there's a fire inside him that wasn't there before. He becomes strong and brave and selfless. He likes to think he's those things already, but when he's wearing that mask, it's somebody else's strong and brave and selfless, not his own. It's like there's someone watching over his shoulder, whispering in his ear and telling him what to do. At the same time, there’s not really anyone there. It’s just him. Just a hero.
Sometimes when he’s wearing the mask, he can remember being the Fierce Deity. It's nothing specific, just a vague sense of self. He'll be in the heat of battle, thinking up strategies to vanquish his latest foe, when suddenly he won't be himself anymore. He doesn't know how to explain it because he still feels like he’s himself, only “himself” is the Fierce Deity and his original self is tucked away somewhere in a corner of his heart.
It doesn't really scare him in the moment when it happens. The transition is always so natural that it never occurs to him to worry about it too much. He is the Fierce Deity. He's so caught up in being him that he doesn't really think to question the things that don’t make sense—the fact that this battle is all he remembers, the fact that he can’t even remember his own name. The Fierce Deity is all he ever was and all he ever will be. It’s all he ever wants to be.
When the battle is over and there's no longer any immediate danger, the mask always fades away on its own, turning back into a simple mask and allowing Link to be Link again. The spirit’s presence leaves him, and it's only then that he thinks to be absolutely terrified. He never knew it was possible to forget yourself so completely, to believe with every fiber of your being that you’re someone else. He always promises himself that he’ll never use that mask ever again. It gives him unimaginable strength, but it’s not worth losing himself and forgetting who he is.
But that mask always calls him back. Whenever he sees someone who needs help, the mask literally calls to him, urging him to take action, to wear it and protect whatever innocent lives are in danger. He never ignores it, not exactly. He always offers his help to everyone who needs it. He just doesn't always wear the mask to do it, but the mask never minds. The mask is happy so long as he helps those in need.
That's why Link doesn't think the spirit in the mask is bad, just strong willed, full of resolve to do what's right. Even though it scares him sometimes, Link is willing to believe the spirit isn't evil. If it was truly evil, it would never let him go. He would be trapped thinking he's the Fierce Deity forever, never remembering himself again. As it stands, he and the Fierce Deity share the same goal. The spirit just wants to help people as much as it can, and in some ways Link admires that.
There's something else, though, something hidden underneath all of the courage and determination. When Link becomes the Fierce Deity, he can feel it weighing on his heart like a dull ache. He thinks maybe the spirit failed to protect someone important, someone he loved, but he doesn't think he'll ever know. He wishes he could have met the man who made this mask, the man who gave his soul to help create it. He wants so badly to play his Song of Healing and give him some peace.
He's tried it plenty of times before. Sometimes after the battle is over and he's still wearing the mask, he'll come back to himself enough to think to pull out his ocarina. Unlike his other transformations, the Fierce Deity’s mask doesn’t transform his ocarina into a different instrument. Maybe the ocarina was the Fierce Deity’s instrument. Link will never know. He'll play and play and play until long after he's back to being himself again, until long after the mask is just a mask again. In these moments, he can sense that the spirit is at peace. The music soothes it, melting away the sadness and the anguish, but the peace only lasts as long as Link is playing his song. Afterwards, he can tell that the spirit is still sad.
The only thing that brings the Fierce Deity long lasting peace is helping people. That's all it seems to want to do. He doesn’t even have to be wearing the mask. Whenever he helps people, he can always feel the mask radiating with happiness. It even works when he helps people with the most trivial of problems. The mask was ecstatic the time he helped that one poor guy find some toilet paper. The spirit reacts to the happiness of others, regardless of how small that happiness might be.
If helping people is the only way to help put the spirit at peace, then Link will help as many people as possible. He'll help every person he can possibly find. He's not sure how many it will take to please the spirit, but it doesn't really matter to him. He'll keep helping people for as long as he lives. If that's enough to help a lonely spirit find the strength to let go, then that'll be just one more person he helped.