Dr. W.D. Gaster isn't a resident of UMH, but he regularly travels there to study the patients and their symptoms. He hopes to cure or suppress their illnesses so they can become part of a normal society again. His hopes are highest for his son Papyrus, whom he visits at least once a week. Sans will occasionally come with him.
Appearance
Gaster's usually wearing a black turtleneck and tan jeans, sometimes with a white labcoat.
Relationships
Papyrus
Out of Gaster's two sons, Papyrus is the favorite. Gaster will visit Papyrus at least once a week to see how he's been doing; if he's done good, he'll take Papyrus with him to an observation.
Sans
Sans and Gaster have a slightly strained relationship. Sans will tend to act like a child when around Gaster (which will cause negative reactions), but he only wants attention.
Asgore
Gaster is good friends with Asgore, and the two have tea and play cards on a regular basis.
Toriel
Gaster is also a good friend of Toriel, and the two can occasionally be caught chatting in the courtyard while tending to Toriel’s flowers.
Alphys
He knows Alphys, but not very well. The two seem to be on decent terms, however.
Undyne
Gaster and Undyne have an awkward relationship. The two know each other, and are nice, but all they know is each other’s names and who they talk with.
Frisk
Pacifist
Gaster loves spending time with Frisk in the Pacifist Run. He sees her as someone who could help the monsters in UMH make a quicker recovery and re-entry into society, which draws him to ask her for help with his research.
Neutral
He doesn’t associate with Frisk much in the Neutral Run, so he has no idea what to think of her.
Genocide
Like Sans and Alphys, Gaster will go after Frisk in a Genocide Run. Although he goes after Frisk to try and convince her to treat everyone with kindness (much like Papyrus does in Undertale’s Genocide Route), and acts disgusted when Sans and Alphys talk about their plans to kill Frisk.
Grillby
He and Grillby are on good terms, as he goes in on a daily basis to get a cup of coffee and chat.
Monster Kid
Gaster doesn’t particularly like MK because of the insults he throws at Papyrus, but tolerates him long enough to do observations
Chara
Gaster knows Chara well, and the two used to be friends. However, after Chara was released from UMH she dropped all contact with him. Occasionally he'll wonder how she's doing, and mention her to whoever he's talking to.
Gerson
He and Gerson are 'friends', as Gerson believes it. The two talk when seeing each other at Grillby's, but Gerson will never remember who he is, even if he writes it down in his journal.
Trivia
~More Info coming soon~