From Twitter
Friends. Nightmare's beta-reader.
Hearts' thoughts on Nightmare
"We've known eachother for quite a while. I'd say we are quite good friends!"
Nightmare's thoughts on Hearts
"Despite what he may seem to someone that doesn't know him, Hearts is a very kind and trustworthy person. I'm glad I can work with a friend like this."
Friends.
Hearts was the first to find him after he got out of the anti-void.
Error lived with him and his brother until he was stable/confident enough to live independently.
Error feels indebted to Hearts.
Hearts cares about him almost as if he was his child.
Hearts' thoughts on Error
"Error is an absolute sweetheart! I'm glad he's able to live a normal life now."
Error's thoughts on Hearts
"Hearts is the only reason I'm able to be where I am today. He's a wonderful person and I owe him so much."