|“||I want to help you...but I can't believe in you.||„|
|~ Underswap!Sans talking to X-Event!Chara|
Underswap!Sans is a supporting character in Underverse. His entire day of unpacking from the underground is ruined when Cross shows up, and when he's done and over with, X-Event!Chara start wrecking havoc on him and his crew.
First seen in the cliff-side looking over the city, Underswap!Sans and Underswap!Chara both unpack their belongings from the Underground, and he explains to Chara regarding those paint canisters. However, they slip over Underswap!Sans's cape, spilling the paint, and that's where Ink!Sans goes through with Underfell!Sans and Classic!Sans.
Confused at first, he recognizes Ink, and he explains that Cross might've went into his universe. He suggested that he went into the city, and as they all traverse there, he finds the queen, having been cut by Cross himself. Underswap!Chara heads over to where Underswap!Papyrus and Cross are, and by looking through Underswap!Sans does Cross gets familiar with his coding. After Ink apologizes to Cross regarding his world's status, Underswap!Sans suggested that he needs a home, and welcome him, which Cross does by almost killing him, which was stopped by Underfell!Sans and Sans.
He tries to tell everyone to actually help Cross!Chara up, but only after they absorb almost all of Underswap!Chara did he started to question him. He's transformed into an amalgamate, and is used to battle it out with Ink before he's reverted back to himself. His belief to believe in someone is broken, and he completely goes all at war with X-Event!Chara. X-Event!Chara pauses it for a moment, thinking he's talking to Cross, but isn't, and they head back with their battle. What Underswap!Sans didn't understand was that he was also harming Underswap!Chara in the process. Once he realizes, he's forced to kill X-Event!Chara, but his brother stops him, telling him they to remember their best timeline.
Once Sans is able to intercept X-Event!Chara's attacks, him, Underfell!Sans, Underswap!Papyrus, and Underswap!Sans fires their Gaster Blasters straight at X-Event!Chara, but all blocked by Nightmare; he kills Underswap!Chara afterwards. He tries to convince Underswap!Papyrus to come with him, but feeling his distrust of Ink, he dares not to go, telling him everything will be back to normal. In tears, Sans carries him over to Outertale, another safe pacifist timeline.
Underswap!Sans makes his first appearance in the episode alongside Underfell!Sans and Classic!Sans. The former reassures him that what his brother was doing was for the better, and Underswap!Sans asks him if he would feel the same if his brother did the same thing to him, to which he replies that his brother isn't too nice. Underswap!Sans says that his brother had never done anything like what he did in the previous episode, and that he had never seen him act so resigned, questioning if that was even his brother the whole time or if it was X-Event!Chara who made him become the way he acted.
Underfell!Sans goes on to describe his feelings while he was being controlled by Cross, but as he mentions XGaster, Underswap!Sans speaks up, claiming that he helped him fight the evil human, who was saying that one of them didn't have good intentions, though he claims that the "Sanses" would never do anything bad. After that, he watches as Classic!Sans heads off to find Ink, though as he teleports back to warn the other two about his findings, Error captures them and ties up their SOULs in his strings after a remark from Underfell!Sans.
When Ink arrives to the scene, Underswap!Sans is the first to say something, desperately begging him to help him and his friends, but becomes worried once Ink doesn't respond. During the battle, Underswap!Sans is forced to attack Ink along with Underfell!Sans and Classic!Sans because of their control under Error, though said control breaks rather quickly once Ink uses his paintbrush to release the strings holding their SOULs together. Later, he angrily confronts Ink about what he spoke about earlier, claiming that he was supposed to wish to protect the AUs and to not interfere like he was doing. Ink responds, saying that it doesn't matter if only a small fraction of the timelines have been affected before proceeding to drop the vial holding XGaster's SOUL and reawakening him, much to the dismay of everyone around him.
As XGaster confronts Underswap!Sans, Underfell!Sans, and Classic!Sans after killing X-Event!Chara, he shows Underswap!Sans and Underfell!Sans their timelines and the monsters in them before shattering both of their SOULs.
Underswap!Sans consists of a gray shirt over-covered by a dark grayish one, dark pants, blue eyes, blue gloves, blue boots, and a scarf that comes out as a cape. As an amalgamate, he's mostly melting.
When he's on his serious side, he'll spawn bones and Gaster Blasters at his will. He's also quick at dodging and grabbing attacks, as shown when he dodges X-Event!Chara's barrage of knife, and slashes X-Event!Chara with one of them when he grabs one.
As an swapped version of Papyrus, he has a belief that everyone is a good person. This is broken after he's overwritten back to normal, and he fight X-Event!Chara even if Underswap!Chara is dying from it. He's relies too much on his brother, so much that he wouldn't go without him, despite leaving Underswap when it was confirmed that he wouldn't come.
In 0.4 however, he seemed to realize the problem much quicker, even going as far as to call out Ink's actions during his and Error's battle, possibly because of the experience and realization the episode prior.
| 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