Tak is an Irken.
She wanted to be an Invader, but Zim had ruined her life 50 years previous to the show after destroying a snack bar and trapping her inside of a structure on Devastis. She then had to wait seventy years to retake the examination of becoming an Invader.
Prior to the show she was training to become an invader on Devastis. Then Zim destroys a snack machine so he can get a snack but this knocks the power of half of Devastis, and leaving Tak stuck in the place she was. This results in her missing her test and being placed on planet Dirt as a Janitorial unit.
When she first appears in the show she is vicious and determined to take Zim's "mission". Her original plan was hollow out the molten core of the Earth, and fill it with snacks as and offering to The Tallest. She tried become an Irken Elite and rise up to being an Invader. When was about to take her test, Zim tried recive a snack out of defective vending mechine, and blew it up. Zim ended up geting his snack, but Tak was trapped in the test cell, missed her opportunity to take her test, and had to wait 70 years to take it again. In the meantime she was placed on Janitorial Squad, and sent to planet Dirt. She escaped and came to Earth to find Zim. She came up with a holographic disguise to blend in with the other Skool children.
Tak has dark purple eyes, and squared curled antenae. She has a small beauty mark under her left eye and a purple outfit similar to the invader uniform. On the left side of her head are implants that are used for mind altering. She can use it for mind control and erasing memories, but it only works on the simple minded. She used it to make Sara apologize for making fun of her poem, and as an added bonus, made her eat her eraser.
Dib devolped a crush on Tak before realizing that she was an alien. She used him to find more information about Zim. But Dib thought she was actually interested in his parmonormal studies.