MettatonxNabstablook is a popular ship for a VERY good reason.

Mettaton is a ghost. Well, was a ghost until he met Alphys and she made a body for him.

His house is actually the one next to Nabstablook's and if you find the key (no, I'm not telling where) you can go inside and read MTT's diaries.

(Hi, admin here, I just want to let you know that you can ask for certain types of facts like character facts or soul facts or facts about areas in the game. Also, don't be surprised that I'm doing a lot of Nabstablook, I love this smol ghost child.)