Fan favorite Firefly star Nathan Fillion is Simon Williams, a.k.a. Wonder Man, in Guardians of the Galaxy Vol. 2.
In the Marvel Comics Universe, Simon Williams is a former members of the Masters of Evil, and brother of villain the Grim Reaper, who reformed and joined the Avengers. Williams eventually retired from the Avengers to pursue an acting career. He was later dragged back into the Avengers’ active roster, despite becoming a pacifist, as a member of the Avengers Unity Division. He paid the ultimate price while defending the Earth from the Apocalypse Twins.