Tazi is an ex-member of the irken military, originally programmed to be an invader.
[Taken from the deviantArt page of her debut]
Ever since she was young she was always looked at and treated differently, like she had some mental illness, or something was wrong with her PAK. It wasn't really that, it was just her abnormal appearance. She originally was programmed to be an invader, but had another smeet's PAK on her by accident. That's why she dropped out of the military early, well, that and extreme injury caused by the fail of "Operation: Impending Doom 1". (Being trapped under a broken building, part tore most of her face off, or at least the skin. She has had half of her face be robotic ever since. Her robotic parts were created from scraps of a broken SIR unit. She was practically an outcast by now, she had no friends. So, she grabbed her things and went out in her ship to a distant planet. It could be anywhere, just not Irk. She came across one planet and landed on it (Earth), and has a small base set up in the middle of a forest. She barely leaves her base, only for extreme emergencies, for the reason she does not have a disguise. She has a small crush on Zim, but he never honestly cared.