'Mii' is the third, acquired roommate. Though female she's treated like one of the guys, at least by Chip anyway.
She doesn’t have ID, and English is very obviously a second language to her, therefore it’s hard to find her a job or get her registered for school. She may in fact be an illegal immigrant.
The guys are pretty sure she’s amphibious, since she drinks copious amounts of water. That doesn’t really explain her hair though. It’s hard to tell from this sketch, but her hands and feet are in fact webbed.