What I would do is not make it DID. Never call it DID and do What you want with your novel. Maybe Nike can not remember the fights because they relate too closely to one of his trauma’s. Maybe it is because he does not want to fight and only let his instincts do the fighting. And at other moments people can heavily swap how they react and act. Want to talk about this trauma? He shuts up and closes from the outside. Want to play and distract him from trauma? He plays with you. Want to have an intulectial discussion on quatum mechanics? He is willing. (These are examples and might not fit the narative and character development.)