He was once the royal scientist, Asgore requested him to journey to the surface as a spy. He followed the orders of the king and ventured outwards only to get caught by human scientists and tested on. The tests resulted in the scars on his skull one running down towards his mouth the other running up towards the top of his skull. Both his palms were drill through leaving holes in both hands, you can basic see through his hands.
He never raised his hands towards humans believing deeply that they were worth mercy and he wanted deeply to show his king the truth of his views. He meets a human named Paul McCartney, he himself is under the disguise of a human and calling himself John Lennon. They created the Beatles and made wonderful music together.
When it came time for Gaster to go back to the underground he was attacked by humans. Paul saved him but ended up dying. this led to Gaster into created Sans. He then went back to the surface and had sex with a woman and gained a son he called Sean.
He ended up going back to the underground for the last time for after telling Asgore all he had learned he went back to the lab and got pushed into the void. His data got corrupted and then rewritten.
Gaster can get very upset if Sans is killed, so upset he glitches and the glitches effect the world around him and deletes it only leaving code behind the only way to fix it is to reset to before Sans is deleted.
Gaster's overpowered and is aware of AUs and can summon clones of himself, he can even summon his very own Error Sans and other AU Sans.
He is aware of Resets and timelines.
They were best friends and writing partners in a band called the Beatles. He died protecting Gaster from a human attack
he is the father to Sean. Sean is unaware of the fact that he is half monster. He protects Sean the best he can.
He created Sans from the soul dust that Paul had left behind. He was unaware of if Sans remembered being Paul or not thus why he calls him Sans. Sans ran away to Snowdin after he woke up in a tube in Gaster's lab.
He and Sans enjoy each other's company and are best friends still and love to do puns together.
As Stated he is able to Disguise himself as a human. He was calling himself John Lennon.
In this disguise he looks like a normal human, it doesn't take much magic to do so and he can turn into whenever he wishes. He enjoyed being with humans and saw them all as friends.
As John Lennon he has long brown hair his eyes are hazel brown and wears the same round glasses as he does as his monster self.