He was a musician and was apart of a band called The Beatles.
He met John Lennon and became friends they both were the creators of the band. He enjoyed making music and silly puns with John.
He found out about John's true form but still viewed him as a friend.
When it came time for John to go, he followed him and saw humans were about to attack him.
Paul got in the way and tried to show them that John meant no harm and tried to spare the humans but they didn't want to hear it.
They attacked and Paul got killed. When dying his soul dust flew onto Gaster's clothes. Gaster then fled for the underground and his lab and thus created Sans from Paul's soul dust.
it is hinted at that Paul was in fact gay and was in love with John
He is viewed as kind and a great listener by Gaster. He saw him as a funny caring man. He loved animals and enjoyed John's company.
When Paul first saw John's true form, he wasn't fearful he was more curious about him. He never did get to fully know him.
When He first met 'John' he saw him as a caring man and was curious. He broke the ice with a pun and the laugh that followed made Paul blush. This is when he realized he liked 'John' more than a friend.
As Monster Form
As Sans he remembers being human and a part of the Beatles. He still loves puns and making music.
He really enjoys Gaster's company even when he was Papyrus and didn't remember his true life.