|“||WE CAN HAVE A GREAT TIME.||„|
|~ Underfell!Sans pronouncing to Cross|
Underfell!Sans is a supporting character in Underverse. When Cross shows up in his universe, and lays his waste all over Underfell, he tries to stop them, but is taken over by Cross himself. After he's freed, he's forced to join up with Sans and Ink!Sans to avoid getting destroyed by his brother.
While Cross marks out parts of his world for the X-Event, Underfell!Sans comes up to him. He isn't too happy over the fact that he's willing to do this, and even though he does gives a reason why he should be involved in his world somehow, Underfell!Sans fights him anyways. Though he's able to impale him, he underestimated the strength that Cross was willing to take, and he clutches back. Struggling, he's suddenly stabbed to follow Cross's control, so that he could be used against Sans. He's tied up by Ink!Sans, but after he's freed, Sans kicks him out using blue attacks. It was enough to free Underfell!Sans from Cross, and before he could deal the final blow, Cross leaves.
Expecting that his brother wouldn't notice, he's shocked to know that his brother actually had him on camera, and informs him that he must leave from his sight, otherwise he'll use his head as spaghetti; Underfell!Sans immediately follows his classic counterpart with that announcement.
While the two of them wait for Ink to come back, Sans begins annoying Underfell!Sans with the echo flowers. With Ink being set free, he paints his side where Cross threw his HACK Knife at. Underfell!Sans is disgusted after Ink is told by Sans that he's coming along, leading him to throw up ink.
Once he's in Underswap, he begins to grow all weepy with their universe's Surface. After they head towards the city, Sans tells him about his plan, and they head off. They were successful at distracting Cross, allowing Sans to get back his soul. But once X!Chara comes back to life, he knocked Underfell!Sans and Sans to X-Tale.
With the two of them trapped there, they're met by Dream!Sans, who tells them to have some positive feelings to escape this place. Underfell!Sans calls his plan "bullshoes", which he turns out horrifically that he was being censored. After Underswap!Papyrus starts reminding Underswap!Sans about this timeline, they both get back into Underswap. After Underswap!Chara is killed by Nightmare!Sans, he pushes Ink towards Outertale when he doesn't respond at all.
Now lodged in Outertale, the 3 Sanses reminiscence on the events previous to them. Classic!Sans offers to find Ink alone while him and Underswap!Sans wait. This wait is shortlives as Sans returns moments later to warn them too late as Error!Sans takes him, and the others hostage.
Now under control by Error, Underfell!Sans is forced to fight for him against Ink before the latter frees them all. Fed up, he blows up the entire battleground that Error and Ink were fighting on for the vial. Underfell!Sans was present when Ink reunited XGaster's dying soul with the one he had in his vial, where after letting X-Tale!Chara live up to his last words as the X-Event, XGaster proceeds to kill Underfell!Sans and Underswap!Sans. Before letting go however, he's petted under Underswap!Asgore as Underswap!Sans is embarrassed to see him.
| 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