When you are suffering from post traumatic stress disorder, can barely function, your mom is emotionally and physically abusing you, and you are on the verge of breaking down further but you get a girlfriend.
This manga does great job of showing just how terrifying familial abuse is. I want more than anything for Sei to get away from Seiko but that's impossible since she's his mom. His home, is her home, and he is her child. He can't escape her.
This is why we should always be nice to people we don't know.
Dude's walking on eggshells at home, his friends don't truly care for him, and his teachers are absolute dumbbells.