He doesn't like puns but likes knock knock jokes, he strives to be the cool brother, and he loves mustard and relish. He isn't flustered and isn't always angry, he's just very tired.

Underfell Sans looks very similar to his Undertale counterpart. He is a skeleton with sharp teeth, with one being gold. His right eye is seen to be glowing red 24/7. He wears a black winter jacket with a jacket fur flame hood and a gray cotton hoodie, and a red sweater with a golden chain with black pants that have yellow stripes going down. He wears red shoes with yellow socks. Sans wears a black jacket, with the "fluff" at the collar being notably more ruffled than the original Sans'. His shirt is a red color in contrast to the normal white shirt. His shorts have a yellow strip instead of a white one and he wears black/red/white shoes or red slippers, as opposed to his counterpart's light blue slippers. His body structure appears the same as his counterpart (besides his teeth).

heres a ref of what he sounds like  
https://www.youtube.com/watch?v=l-MuEimQcKk&t=69s