They care so much about each other. They resent each other. They trust each other. They don't trust each other. They don't want the other to ever leave their side. They feel like the other's presence is suffocating. They want the other to know everything about them. They can't let them know anything about them.
I did a scene redraw of dmc 4 cuz the way it cuts to Dante with a sword stabbed in him after Nero asked that he's not human was just kinda funny to me.