Prisoner 775

Prisoner 775 is the main alien, villain and titular character in the episode Prisoner Number 775 is Missing.

Backstory
Prisoner 775 was exiled from his homeworld by an oppressive dictator and crashed on Earth where he ended up imprisoned in the secret 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 advising him to "seek justice"), but was stopped by Ben as 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. He said that " the revolution was lost, my mate, my children gone, all gone, there is nothing left to live for". When he heard the word "justice", he said he wanted to locate and destroy Colonel Rozum and his family for what he did to him.

Trivia

 * When Prisoner 775 turns invisible, the way his mouth and eyes are clear for a while as the rest of him disappears is similar to the Cheshire Cat in Alice in Wonderland.
 * Both Prisoner 775 and ChamAlien bear a strong physical resemblance to Disney Pixar's Monsters Inc. character, Randall Boggs, all three being reptilian, based on chameleons, and being able to blend in with their surroundings.