Mirage is not revealed to have any super-powers and is a supporter of Syndrome and his hero-killing agenda. Her reasons for lending technical expertise to Syndrome are never reported. She has killed heroes with Syndrome so she is not opposed to killing but did not believe in the indiscriminate killing of innocents or children.
Mirage and Syndrome have a cordial employer-employee relationship of sorts but it appears Syndrome would like it to be more. Mirage is a capable computer scientist (and possibly covert agent) and is the only other person to have access to the Kronos database outside of Syndrome.
Any respect Mirage had for Syndrome was lost after he threatened her life.
Well, Syndrome loves to kill the "Supers". If Mirage has power, I believe he would have tried to kill her (as he'd done to other Supers). I think she just happened to be his girlfriend and a trusted personal assistant.