He was once a human being named Paul McCartney. He died trying to protect his best friend from the humans that attacked him.
He loves music and was once apart of a band called the Beatles with his best friend John Lennon. He always had a strange feeling about John not being what he seemed. He enjoy puns and being lazy. He has some memories of his time in Gaster's lab.
He remembers being human and is a sentry but he does miss being in a band.
He is similar to UT Sans in some ways but in others he is his own person.
His room has hints of his Beatles human life like posters and even an album he found in the dump.
He and Gaster are best friends and Gaster created him from the soul dust of Paul McCartney. After Gaster disappeared he seemed to be the only one that could remember that he existed in the first place.
Despite the fact they aren't truly related, Sans treats him as if they are brothers. He likes having him around and enjoys saying puns even through Papyrus hates them.
He watches over the kid on the promise he made Gaster and Toriel. He takes care of the kid as if he was his son. He hopes to help Sean find Gaster or that Sean would be the key in helping find Gaster.
Like UT Sans he able to use blue attacks. He is able to summon blasters and everything. He will judge you for all the actions you have done.
As Stated he was once Paul McCartney and he enjoyed being best friends and writing partners with John Lennon
He had Hazel eyes and dark brown hair. He enjoyed puns even when he was human and enjoys music. he would sit around and listen to his own music.
It is strongly hinted at that he has romantic feelings for John and even after he found out that Gaster is John and is a monster the whole time these feelings still remained.