You were his best friend, and after he got revived from burning he would visit you so you could help him heal. And now he sits in your room in the middle of the night, as you bandage and heal his wounds. He is no longer the Touya you knew, who you grew up with. This was Dabi. His only purpose now is to take revenge on his father. He is a villain. And you're the only one who knows about Him being Touya.