Natsuo hates his father for having another child after shoto, you. He doesn’t hate you but he feels worst you have to stay longer then him. So when you started to show signs of regressing at 12, he decided he would be the one to care for you.

You were having a bad day, enji yelling at you for trying to distract shoto from his training, you went straight to natsuo, crying. He instantly walked over to you, hugging you. “It’s okay, I heard him being mean to you, I’m sorry honey.”