Steve Willits

The Willits Imp. Instead of throwing fireballs like in the games, the film Imps could fire it's tongue at targets. The tongue seems to be the primary method of infection, the tongues usually seen being targeted at the character's necks

Steve Willits was a scientist working for the UAC on Mars. His wife is Dr. Jenna Willits. He was mutated into an Imp that killed Goat, and as a result was killed by Reaper. His body was later dissected by Samantha, in which case she discovered that the monsters are human.

The character's name is most likely a nod to Tim Willits, id Software's creative director.