well, I don't know either. But I'm guessing Joy turned out the way she is... is b/c her mom doesn't seem to love her, & we know why. it's b/c that lady isn't her mom & her grandma probably doesn't love her, too. So I'm not very surprised she is the way she is. Maybe this is why Nott likes her... not just for her looks... maybe more b/c of how she is resonates with how he feels. Being looked down upon by a stepmom, seeing their sibling loved more, etc....
She may seem confident, but really, it seems like she has low self-esteem which leads her to be the way she is. There's also the rape child by chatchai. I'm pretty sure his mom isn't so kind to him either, & he doesn't know why. At least, he has those 2 guys raising him instead w/ a lot of love.