Fell: Fell!Sans is lazy like Classic!Sans, but he is shown to be more active, rude and brutish. He has a semi-what hostile relations ship with his brother and is incredibly foul mouthed.

Ink: In the long run, Ink!Sans is generally optimistic, has a sense of kindness, loves making art, and can get too excited even to the point of throwing up ink. In the more serious side, he objectively tries to reason with anyone, and only follows who he finds his best friends. However, he has no soul, and relies heavily on emotion vials.[3][4] If he stops taking the emotion vials for a certain period of time, he will lose all emotion that he once had and is unable to do any sort of action, as shown in Underverse 0.3 Part 2. This doesn't matter though, as Ink intended to not use his emotion vials for his plan to revive his friend. Overall, this proves him as being villainous and selfish without the "emotion vials".

Cross:At first, Cross was far more compassionate as the original Sans, . But when Frisk took control of their friends, including Cross' brother Papyrus, he lost all faith in him, and used all of his power to stop him by killing his former friends, even ruthlessly killing Papyrus and absorbing Frisk's soul. His greed and recklessness was shown as even when Chara told him he couldn’t overwrite, he killed Frisk anyway. Once he realized the truth, he had a mental breakdown and became completely delusional over the loss of his own world, even to the point of throwing his negative feelings at Chara.Despite this, he becomes excited and obsessive over tacos and chocolate, and was very friendly towards Ink before he told Cross that he couldn't revive the dead. Afterwards, he acts arrogant towards others and is disgusted by everything around him. Cross is also more active than the original Sans and was never lazy (EVER). He also became very selfish, ruining other AUs pacifist timelines just to restore his AU.

Sans: Originally, Sans was laid back and lazy due to knowing that whatever progress he made would be reset by Frisk. However when he becomes the new owner of the timeline after killing Frisk in a way he wasn't supposed to, he starts to become more outgoing and experiences more things now that they cannot reset without his consent. Despite this he is still is quite lazy, most likely because it's grown on him.

Xtale!Chara:Once XGaster forcibly takes half of his determination and corrupts him with half of his OVERWRITE soul, X-Event!Chara has become much more sadistic, even killing his own brother several times in Timeline VII. However he still loves his brother convincing XGaster to let him revive XTale!Frisk. Ever since he OVERWRITTEN himself into XTale!Frisk and became the new X-Event he played the role a master puppeteer manipulating others for his own gain.

Nightmare!Sans: Nightmare!Sans is, in general- sadistic. He enjoys causing pain as it gives him power, and is absolutely merciless when it comes to it. Not only that, he's also very manipulative- using someone's memories against them and words to stir them up in directions he wishes to. While he tells Cross not to kill anyone, he's given X-Event!Chara a chance to kill somebody, and when that fails, he kills Underswap!Chara himself. He even tries multiple times for Underswap!Papyrus to die. Nightmare!Sans also hates broken deals, and would destroy a victim's life for that.

Underswap!Sans: As a Sans but with the personality of Papyrus, US!Sans is kind, energetic and has a belief that everyone is a good person. This is broken after he's overwritten back to normal, and he fights X-Event!Chara even if Underswap!Chara is dying from it. He 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.

Error!Sans: Error may have a sensitive feeling towards others, but he's often shown to be remorseless, wanting to destroy all the AUs Ink cared about so much. Even though the truce was made, Error still had a negative feeling with Ink, especially when he started talking about his new friend. He even extracts Cross's soul without Ink's consent, but he still decided to keep it intact to stay loyal with the truce. Once he makes the discovery, though, he feels confident that he'll beat Ink and destroy his precious AUs.

XGaster: XGaster is more of a mysterious character, showing his isolation to the main characters except in special times, such as when XTale!Sans attempts to murder XTale!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 safekeeping. Despite this, he shows much more borderline sociopathic and abusive tendencies towards his children, seeing them more as "experiments" than anything else. He barely cares about anyone's else feelings, and only wants them to fall into his own hands, using them as a puppet to his advantage. He is also shown to be extremely obsessed with control and shows several traits of narcissism such as ranting about his kids for fighting back and accusing them of being "ungrateful." He also calls the other universes that aren't his own "garbage." This all shows that beneath his stoic and calculating demeanor, he is mentally a child who sees his universe as a toy box.

Fresh: In his Cameo, he appears to look like the same as he appears in CrayonQueen's game "Lucidia." The parasite known as "Fresh" is seen possessing mostly one skeleton, which is medium tall and has thick bones. Some details are the same no matter what they are possessing, such as Fresh's attire, the thick black eyebrows, and gold tooth. Fresh's attire consists of a pair of thick black sunglasses that say in aqua and yellow "YOLO" across them, a red cap with an aqua propeller and yellow bill, a pair of aqua fingerless gloves, a light pink shirt with a v-neck and notched collar, a purple jacket with a white and aqua stripe across the top, the white stripe is smaller and has an aqua squiggle through it, and an orange collar with purple lining, a pair of long light green shorts with a thin white stripe down the side and a removable bottom that is light blue separated by yellow zippers, a red fanny pack a with green stripe across the top and "SWAG" in white lettering bellow, a pair of light pink socks and a pair of white, light up shoes with aqua circles on the inner side near the back and purple and yellow soles, orange tongues, and red Velcro laces.

Dream: As a byproduct of positive energy, Dream display a quirky, yet unconfident personality. While he is too talkative and over-dramatic, he does give concern that something terrible could happen and, in response, tries to settle down the issue. For example, Sans begins to feel that something is wrong with Ink, but his conversation is cut short by Dream.

Epic!Sans: Is notorious for being humorous in spite of how serious the stories that he's involved in can be, whether it be fighting a corrupted child, or his own father. A very popular catchphrase that Epic!Sans prefers to use is "bruh", something that Cross likes to supplement with "dude". Despite this, he's very active, yet chill about doing his job as a royal scientist, and is willing to put himself under the risks to take out his enemies. He also likes reading memes as a hobby

Killer!Sans: has a psychotic, cocky, and hostile personality. He often kills monsters in his path, and usually has a child-like attitude when he “loses.” But he as well is very protective of his past, which is not heard (except by him) of in Underverse. he appears to have a penchant for knives

Fatal_Error!Sans:Originally, he displays indifference towards those he encounters, remaining stoic until someone interacts with him. When engaged in conversation, he tends to become nervous. People who show him kindness often trigger memories of his beloved Papyrus. However, he ultimately recognizes that these are not his closest person. Despite the harsh reality, he clings to his hope that his Papyrus is still alive. His relentless quest involves collecting or copying the bodies and code of Sanses and Papyruses from various Alternate Universes, all in the hope of reuniting with his Papyrus and reconstructing his code. In Underverse, he was portrayed as a destructive force, targeting others. Presently, his primary objective is to locate XGaster and exact his revenge by destroying him.

X-tale_Alphys: Post-OVERWRITE she is cold and apathetic. She is uncaring towards anything outside of her purpose, to be XGaster's assistant. She retains some basic traits, such as an attraction to Undyne and an inability to connect to others. She seems to struggle to express any emotion, but retains the ability fundamentally.