Scene stealer

Usually a stock character in a film: sidekick, best friend, confidant, villain, etc., unofficially declared a "scene stealer", which is someone who steals the scenes they're in, particularly for charm, and the ability to suck in an audience's attention.

More notable scene stealers include Steve Buscemi in Reservoir Dogs, Samuel L. Jackson in Pulp Fiction, Thelma Ritter in Rear Window, or Dan Hedaya in Clueless to name but a few.