Doop
Doop was the most mysterious member of the media-obsessed mutant team X-Statix, and not just because of the unique language he spoke. His history is unknown, but it's believed he was a product of a U.S. Cold War experiment to use against Russia.
Doop was said to be tremendously powerful, but never fought in any of the X-Statix missions. Instead, Doop served as their always-present cameraman, taping everything that the X-Statix did.
Despite being mysterious, hard to understand and looking like a floating green potato, Doop was a popular member of the team, and could be seen on lots of merchandise. Doop was believed killed in the ill-fated last X-Statix mission.