Mortar is an optional boss fought during the story mission Toil and Trouble. Killing him is optional, and thus he can be ignored. Killing him will fulfill one of the mission's optional objectives.
Mortar's most prized possession is a custom buzzard, which he calls the Boombringer. He is never seen flying it, but Boombringer is destroyed in order to anger Mortar and trick him into bringing the elevator down.