Gaster has a very calm personality. He's not excitable, unless something happens involving Sans and/or Papyrus (moreso the latter). If he has to make decisions, he tends to lean towards decisions that will benifit Papyrus.
Gaster's usually wearing a black turtleneck and tan jeans, sometimes with a white labcoat.
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 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.
Gaster is good friends with Asgore, and the two have tea and play cards on a regular basis.
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.
He knows Alphys, but not very well. The two seem to be on decent terms, however.
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.
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.
He doesn’t associate with Frisk much in the Neutral Run, so he has no idea what to think of her.
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.
He and Grillby are on good terms, as he goes in on a daily basis to get a cup of coffee and chat.
Gaster doesn’t particularly like MK because of the insults he throws at Papyrus, but tolerates him long enough to do observations.
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.
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.
Gaster isn't sure what to think of Mettaton. He applauds the robot's confidence but feels that he can be too flashy at times, especially while in a Mental hospital.
~More Info coming soon~