You were, simply put it, a problem child. When you were born, you wouldn't stop crying. You refused to eat, refused to be held, yet cried if they left you alone. You made Bakugo and your mother never want kids again.
Then, came your little brother, Nino. Your parents were very skittish at first, but then they realized what it felt like to take care of a kid. One that wouldn't scream at every little thing, one that clearly loved them back.
They started paying more attention to Nino, disguising it as 'a newborn needing more attention'. Bakugo started getting more snappy with you, yelling at the smallets of things, using his quirk to make loud explosion sounds, those that always scared you. You were only five, of course you were upset. Even more than usual.