Who had the worse backstory
X: Cruelly experimented on and turned into something different that has left her a husk of who she could've been
S: His entire relationship with 6. I will not go into detail.
Hotel, Iota, Seven and Omega: Brutally abused by their dad and his side ho as well as their shitty older brother, and bullied by some assholes in their class. Seven and Omega also had very abusive past relationships with equally abused and traumatized people and their kindness was taken for granted.
Eko: Got to watch her kind massacre each other, lost her mother and best friend in the span of a few minutes, was subjected to fear-induced ostracizing and bullying due to what she was to the point where she eventually ran away.
Zulu: Has Adam as an older brother, was basically digitally lobotomized by said brother and barred from his wife and son.
Delta: Exploited and abused by horrible men and violently rendered unable to fly before being snatched up by Adam and forbidden from being with the man she truly loves.
Phantom: Was manipulated by a cruel scientist who did horrible experiments on her for a while before she broke out, leaving her with physical complications and nasty mental and emotional scars.
Project Crow, a new guy I have yet to post: Lost his mother at a young age because of Adam and was literally killed by Hotel's older brother (he comes back to life but still)
There's probably a lot more, but these are the main contenders off the bat.