Trauma! 

Edelgard: “She and her siblings were experimented on by a group of people called Those Who Slither In The Dark, which caused everyone but her to die. She then was forced to work with that group until she was safely able to deal with them years later.”

“She and her siblings were kidnapped and experimented on when she was 9. She was the only survivor, and the experiments and torture turned her hair white. Even after the torture ended, she still couldn't escape the ones who did it to her and has to scheme and work alongside them until she is powerful enough to snuff them out”

“she had horrific like magical experiments on her as a CHILD that is heavily implied to involve slicing into her flesh and probably doing blood maybe even BONE TRANSPLANTS and this CAUSED HER HAIR TO TURN *WHITE*”

Cellbit: he was kidnapped by the Federation (evil organization) and forced to solve enigmas 

Propaganda!

Edelgard: “She is defined by her drive for societal change, and she will do whatever it takes to make sure no one else suffers the way she did (even if that means starting a continent-wide war and working under the thumb of those who tortured her until she can backstab them). Also her (especially post-timeskip) character design is the coolest thing ever.”

“LOOK AT HER HER HAIR IS WHITE I WILL NOT TAKE THIS INJUSTICE for real dude what are you talking about?????? Also notable Lysithea from the same game also has white hair and also had the exact same type of magical experiments done on her as a guinea pig for what they did to Edelgard so that's just a thing putting a second Crest in people turns their hair white ok listen to me” (note from Mod Lars: omfg, I’d forgotten about this entirely until I saw it! this is my own submission from long before I was a mod on here, and was made in a fit of rage because Mod Pure was saying her hair isn’t white 🤣 Also, Lysithea lost to Cecil RIP)