how do you want to do this?

Not a single one:

Soldier Boy: I taught her how to jerk off.

Andre, Marie, and Jordan:

If you want to write a character with an unspeakably awful past, there’s no need to go into deep and gory details about how horrible it was. Readers who can’t relate to it won’t relate to that, and the readers who have been there generally don’t want to see that. Instead, highlight some of their happiest moments but make them unsettlingly small.

Sprinkles in some realism, too. Having a character go “my parents were abusive monsters and I’ve literally never had a happy moment in my life” isn’t realistic, and both the people who haven’t witnessed that kind of thing outside of fiction, and the people who have personally lived it will just go “yeah, yeah, tragic childhood, misery, darkness, we’ve all seen it”, and being nothing but negative makes the character both uninteresting and unlikeable.