Horror

He's a skeleton. That's all.

He has a hole in his head, which appeared from an accidental fall on the reinforcement, which has little effect on his sensations (auditory and tactile), even increased them.
(in the original Undyne gave him a lobotomy, but here she doesn't, so who cares)
Below the hole is a red eye that takes up the entire eye socket.

Appearance: wears a regular sweater with rolled-up sleeves, dirty flesh-colored.
There are shorts with two white stripes on the sides, and black shorts.
Sometimes he wears pink slippers, fluffy ones, but his are dirty pink.
Socks are grey.