What should I add to these characters' backgrounds?

I'm writing a fanfiction, and some of these characters are boring and don't have much to them. What should I add to them? It's an MHA and LoZ crossover because I'm weird like that. The first one for example, seems complex enough for now. I might add more as ideas come to mind and as thing unfold but yeah. I want the other characters to be just as complex, if not more.

Example: Suri
She's a student at UA. She's closed off, quiet, blunt, and sarcastic. She makes it clear that she didn't come to UA to make friends, but to be the best she can be, for intentions darker than her fellow classmates expected. She's a protagonist, but not the main one, and she kind of seems like a mini antagonist, but she's really not. Her left hand, up to her elbow is blue, due to her quirk (power) which is essokinesis, but given the title Reality. So with the touch of her body she can make things disappear from existence or bring things into existence. She basically manipulates reality. She wears a Keaton Mask all the time and never shows her face. Her left ear has a tear through it, and is completely deaf, though no one knows she's deaf in that ear until later. The reason the ear is deaf is because of an accident with a villain that happened not too long before she went to UA. Her goal is revenge. She wants revenge on the villain, and forgets that being a true hero means to not let corruption control you and drive you away from what is good. Overusing her quirk can cause lack of reality and makes her unable to think straight for a short while. She will be confused and attack whatever she thinks is the enemy. The person to inherit her power before her went insane due to overuse.

Ye. She's got some nice substance, I think. These other characters have nothing.
4 mo
1.) Naydra
Named after the ice dragon from BotW. Her quirk allows her to use anything as a weapon and flawlessly. She's got a kind, good heart and is levelheaded. She doesn't say anything out of line.

That's it. She needs more flavor.
4 mo
2.) Unchi
Happy, annoying sometimes. Her quirk allows her to manipulate earth elements. Geokinesis, that is. Her sister is depressed and tries to end her own life. Downside to her quirk is that it can make her dizzy and tired.
What should I add to these characters' backgrounds?
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