The secret Identity and history of Mirage is kept secret by the government.
In the movie, Mirage is the personal assistant of Syndrome and is tasked with finding superheroes to test his Omnidroid prototypes. When she first appears on screen, she is pursuing Frozone, but sets her sights on Mr. Incredible after his display of heroism.
She then contacts Mr. Incredible and presents him with the same dilemma that she gave the other heroes, he needs to stop the Omnidroid, which had become so smart that realized it didn't have to take orders. Mr. Incredible completes the task and continues to do tasks until he can't complete the updated version of the Omnidroid, is captured by Syndrome, and realizes Mirage's involvement in the scheme
Thinking his family killed, Mr. Incredible grabs and threatens to kill her. Syndrome, knowing he's bluffing, tells him to go ahead and do it. Mirage then realizes Syndrome doesn't care about her life saying "Next time you gamble, bet your own life."
Later on Mirage sneaks into Mr. Incredible "holding cell" an releases him. He attempts to strangle her, but she explains that his family is still a live. Mr. Incredible is ecstatic and hugs her.... just as a suspicious Mrs. Incredible enters the room. Mirage attempts to introduce her self and is promptly punched out.
Towards the end of the movie Mirage is seen instructing the family on how to counterattack the plan of Syndrome, from some intelligence room.
Mirage is an attractive woman, with an almost devious voice. She is sleek and intelligent, but also able to pretend to be genuine and even sympathetic. Apparently she was never always bad, because she does help the Incredibles and by extension the world