Black is so interesting to me. He's in so many ways Red's opposite which makes sense, but the one thing that manages to unify them is *guilt*. Specifically guilt about actions that have harmed Frisk. And while Red tried to deny it where Black almost immediately let it consume him they both ultimately let that guilt drive them to the same conclusion- to follow them.
Also!!!!! Also the fact out of all of them Black is the most Frisk's *parent* (and the fact Red and Fell both have arguments for second place but that's another discussion) just makes me feel things. That's his baby!!!! That's his baby and they keep ending up horrifically out of reach oh my god. Honestly if Toriel ever gets over how they met the two of them would probably be really good at sympathizing with eachothers plights.
Anyway I want to lock Black and Red in a room and not let them out until one of them stops moving just to see who wins
oh absolutely, I think you're seeing something vital to each of them here. They both blame themselves, but Red decided he needed to seek retribution by torturing himself, and Black decided he needed to devote his life to making up for what he did in any way possible. And, interestingly, Black killed Frisk where Red refused to...but yet he's still the one who Frisk trusts not to betray them. And Red's still the one who can make some part of them feel like it's safe to relax.
Black and Toriel would make great friends tbh--Black has Issues with parenthood and the concept of being a parent, having killed his own parent to save his and Slim's lives; but I think that Toriel would be able to recognize him as a young, unsupported father to a traumatized kid who had to make terrifying sacrifices to keep them safe. And since so much of her relationship with Frisk is characterized by providing them the support they need to be functional but not understanding how to make them feel safe, Black's obvious familiarity with their trauma and the automatic adjustments he makes for their sake make him seem like an ideal parent. I think they'd also have a lot to bond over wrt each of them being afraid of losing a child, and having grieved over lost children. Toriel would be very happy for Black that he got his back.
Also unfortunately Red survives the battle royale because he's not about to let Black kill him. that's for frisk to do, obviously.