Prisoner 775

Prisoner 775 is the main alien and titular character in the episode "Prisoner Number 775 is Missing." He is voiced by Dee Bradley Baker.

Backstory
Prisoner 775 was exiled from his homeworld by an oppressive dictator and crashed on Earth where he ended up imprisoned in Area 51. After Area 51 is visited by Old George, Prisoner 775 escapes. He wanted to kill Colonel Rozum and his family for his association with the air force for imprisoning him for years and letting his family die (although Kevin Levin suggested him to "even the score" with Ben Tennyson advising him to "seek justice"), but was stopped by Ben Tennyson in the form of Ultimate Wildmutt. Prisoner 775 was handed over to the Plumbers for interrogation as his DNA is scanned by Ben's Ultimatrix to unlock ChamAlien. His whole family is dead.

Trivia

 * He sounds a little bit like Klaus Heissler from American Dad, as they are both voiced by Dee Bradley Baker.
 * He became paranoid after being locked in Area 51 for a long time.
 * It is unclear how he knew about the fate of his family, given that he was imprisoned at the time of their deaths. Perhaps there not dead and his paranioa got the best of him.
 * When he turns invisible, the way his mouth and eyes are clear for a while as the rest of him disappears is similar to the Chesire Cat in Alice in Wonderland.