COTVGore take.

He is being held by WD.Gaster during his experiments to the DT Extractor for 12 months, he is a necromancer and a summoner, he will create a army of the mutant monsters including summoning Papyrus and Undyne, he's dwells in a old laboratory, which is never ends, due to his condition, Sans was waiting to drink the Poison of Light to take Gaster from his world, otherwise the destruction war is going to start soon.

Sans - Dr.Victor

Gaster - Bebal Behemoth