This is how there relationships is. Female Deku and male Netsu were just so kind and joyful but Netsu was like a father kind and caring but ever since the incident and all the bullying and people constantly using her friendship to there own advantages Netsu became like a chained up beast, Cruel, Overprotective, aggressive, envious and bitter. He constantly chase people away from Izumi so people don't hurt her and Izumi shut down and was depressed and lonely. They did deeply care for each other but in a sort of twisted way. Netsu would constantly cause pain for others and would sometimes hurt her cause he uses too much of her fire which burns her and Izumi hurts him by putting the collar on. He forced to stay put and only tiny parts are allowed to come out. Imagine like needles being forced to keep you down. Yeah it like that. This is when they realize that they are both hurting each other and they are both apologizing to each other for everything. Netsu for going mad and Izumi fir forcing him back.

The collar

This is what this scene looks like after Izumi attempted suicide