|“||LET'S SEE WHAT YOU'RE CAPABLE OF BEFORE KILLING YOU, EXPERIMENT.||„|
|~ XGaster telling his experiment|
XGaster is a major character of X-Tale and the main antagonist of Underverse. Failing to create a perfect world, he suddenly gets an idea to create one once he's shown the multiverse by Ink!Sans. He uses each idea from each AU into his universe, but his constant experimentation meant his own creations started to rebel against him, leading to his creation of the X-Event and subsequently watched over each and every thing his experiment does, even if it's outside his own universe.
- See also: XGaster/Entries
XGaster is first greeted by Ink after his agreement to Error!Sans's truce. He and Ink became friends, with the latter showing him the multiverse for any ideas for his universe. He soon gets approval by Ink to start his own experiment regarding the Overwrite button.
He first starts his experiment by allowing X-Tale!Frisk and X-Tale!Chara to age and then killing their loved ones. He tries to a swapped version of X-Tale and a genocide run, but both were deemed a flop to XGaster; the latter especially failing since one of the kids stopped the other from killing. In the fifth timeline, they both go against XGaster, but he eventually gets control of X-Tale!Chara, who rebels against his brother. He soon gets fascinated with his X-Event, seeing their creative abilities.
XGaster acts as the father of X-Tale!Sans and X-Tale!Papyrus, and acts ignorant towards them, even ignoring the taste of X-Tale!Papyrus's spaghetti. After X-Tale!Frisk forces X-Tale!Sans to kill him, XGaster intervenes, and eventually resets to have it go the way it should. Once the party begins, however, he is struck back after he finds his X-Event missing. Once X-Event!Frisk returns with X-Tale!Sans, he arrives to impale the two of them, threatening reset as well.
However, this didn't erase the memories of all of his characters, and they got him back for manipulating X-Tale to his likening. When XGaster refused to give the X-Event the location of his other half of the Overwrite button, he loses his legs to them, and once Cross comes up, he shoves him out of the hall towards a forest, where he meets X-Tale!Alphys and tells her to store his entries. He slowly fades away, evaporating the universe, waiting until the X-Event shows him something amazing for once.
XGaster makes a brief appearance in Part 4, where after Sans is knocked out from the volleyball Undyne slammed.
Sans soon talked with Cross over what he's seen that day, and would unleash at them if he found out he's in league with them. He also makes a brief appearance before the episode starts, hinting to the next episode.
XGaster makes multiple appearances throughout Underverse 0.3. He's seen from a crowd of monsters when Cross slit Underswap!Toriel's right eye. He appears once again when X-Event!Chara is about to impale Ink, overwriting Underswap!Sans and Underswap!Papyrus; he appears again when Underswap!Sans tells him what has harmed X-Event!Chara won't heal him; he's also seen side-by-side with Ink when X-Event!Chara tells Underswap!Sans that he's mad at him; and he makes multiple comments throughout the entire battle.
XGaster appears again in Error's dream; after Ink eliminates Asgoro and Sins, XGaster proceeds to alarm Error that this game is getting more interesting, and that he needs to wake up.
In the Truce, he wears a coat consisting of spikes on each side, a scarf, and a pair of glasses. He drops his scarf by the start of the experiment, and he now bears a X strap around his suit and shirt, and a belt with a X strap around his pants. Thanks to the Determination he gained from X-Tale!Frisk and X-Tale!Chara, his appearances in Underverse is a semi-melted version than his real self, and, thanks to X-Event!Frisk, has no legs, which doesn't matter as he can still float around to move.
XGaster is the sole creator of X-Tale, allowing him to use the Overwrite button at will. However, he loses some self control over it after he sent half of his soul to the X-Event. This allows him to reset, but destroys any chance of using the Overwrite button. His reset button doesn't even restart the whole timeline again; it brings back everyone to their normal position, but fails to erase any memories that the characters had before.
XGaster is more of a mysterious character, showing his isolation to the main character except in special times, such as when X-Tale!Sans attempts to murder X-Tale!Frisk on stage. He's also a very clever man; knowing that everyone would soon revolt against him, he took his own soul and handed it to Ink for safe keeping. Despite his knowing mysterious behavior, XGaster is known to manipulate subjects to do his work (i.e. telling Ink to eliminate the X-Event).
In the season finale(and subsequent comics after the reveal) he shows much more borderline sociopathic and abusive tendencies towards his children, seeing them as "experiments" rather then children. As well as having Ink!Sans manipulate the main cast. Despite ressurecting Cross because "what happened wasn't his fault" he then Shows he is offering to join his army or continue being dead.
That said he displays atleast ONE moment of genuine kindness when he teleaports sans to his main universe
| X-Tale: Cross | X-Event!Chara | X-Event!Frisk | XGaster | X-Tale!Papyrus | X-Tale!Undyne | X-Tale!Toriel | X-Tale!Asgore | X-Tale!Asriel | X-Tale!Mettaton | X-Tale!Alphys