PLEASE NOTE: THIS CHARACTER IS SPECULATIVE. ALL INFORMATION ABOUT THIS CHARACTER IS NOT CONFIRMED.
There are two depictions of the Underfell Gaster, one being more like most Underfell characters having a dark and cruel personality with a darker appearance, and the other being his moral opposite, so I'm only showing you his actual opposite, but if you feel obligated to add his other depiction, I'm not going to stop you.
W.D Gaster was the former head scientist, it's unknown whether or not in the Underfell comics he's dead or was just kicked out for being soft. In one depiction of him, he is not a scientist..
Gaster, unlike his Undertale counterpart, does not follow but is there if they need him. He is a very quirkcharacter, being an optimist and is willing to help anybody, but he is extremely hyper-active and oblivious, making him obnoxious and sometimes getting in the way.
Gaster is an evil, dark, and cruel entity scattered across space and time. Like most Underfell characters, he has darker and more evil personality than his Undertale counterpart. Gaster was also the previous royal scientist, before Alphys took his place after the 'incident.'
Despite him being a Fell monster, he's not evil, he's just a jerk. He's mostly a jerk to his sons, such as laughing at them for being scared of thunder. He does sometimes show a soft and caring side, shown when he sang a lullaby for his two sons, to help them calm down. He is closest, out of all the GasterGang, to Blindy. He has been a Prisoner of War, and had escaped 3 days after, at the cost of his pride. (story can be found here.) The war had given him Post Traumatic Stress Disorder, otherwise known as PTSD, and fireworks will set off his flashbacks if he does not take his medications. An incident with Muffet had given him Arachnophobia. He has to take 3 different medications for his PTSD, one to prevent, or at the very least, minimize his initial reaction, one to help his occasional depression and one to help him sleep, if he is having trouble sleeping.
Gaster looks no different from his counterpart, the only difference being his face is more melted and mangled slightly to match along with the rest of the game's theme
Supposition about him
WARNING! This is nothing but speculation. DO NOT TAKE THIS AS TRUTH!
Gaster is probably the creator of Sans and Papyrus, explaining their connections. But it is presumed that once the new rule, "Kill or be killed", was added, Gaster's creations became very hostile towards him. It is unclear how he lost his job, but some possible reasons would be:
- The same thing in Undertale happened in Underfell, resulting in him becoming forgotten forever.
- He was murdered by Sans and Papyrus, even though Gaster is (probably) their father.
- He was kicked out of his job for being too soft.
- He was murdered by Alphys, explaining why she got the job.
- He just gave up and went into hiding, because he was sick and tired of all the nice people he once knew being so evil and corrupt, especially towards him.
- He was murdered in a riot, because the rest of the underground thought he was too soft.
- He got destroyed by his latest creation.