Mirage is what humans call "old money." Before the war, he was a member of affluent Cybertronian society, with his own large estate and high-priced friends. He enjoyed the finer things in life: vintage energon, swank evening events, and turbofox hunts on the grounds. But when the war started, he was forced to choose sides and found himself banded with a group of commoners he didn't really understand.
Some of the other Autobots don't trust Mirage. He feels the war, even after all these millions of years, is beneath him. War is not, after all, how civilized, erudite beings conduct themselves. He often questions the Autobots' violent means, and not-so-secretly hopes they and the Decepticons can get along. Because of this, he has been labelled by a few others as a Decepticon sympathizer. Still, he sided with the Autobots for a reason, and he is there for the Autobots when they need him.
Although the details vary by continuity, Mirage has the ability to create illusions which typically either render him invisible or make him appear to be somewhere he isn't. Source: tfwiki.net