A joke villain covered in holes, who works out during the film exactly how frightening that is.
Johnathon Ohnn is a former Alchemax scientist turned supervillain after an accident, whose body is now covered in interdimensional portals that let him travel through space and between universes. Jason Schwartzman voices him.
He was not originally meant to be the main antagonist. Phil Lord and Christopher Miller dismissed the idea when the producer Avi Arad suggested him, and changed their minds once they saw what the power could do on screen.
Minor spoiler
The accident is the collider at the end of the first film, which means Miles made him. The film’s whole thesis is about consequences arriving late, and the villain is one of them.