Patient Numbuh C, or "(the) C.O.O.T.I.E.S. Girl", was a blonde girl who was quarantined in the Deep Sea Science Lab by KND scientists who were convinced she had C.O.O.T.I.E.S.. Cree, looking to cause trouble, set her free, and everyone in the lab fell into panic. Sector V was called in to deal with her, not understanding why she was considered a threat at all. Patient C. is rather uncreepy, saying things like "Hello" and "You're funny" in a very unominous manner.
At the end of the episode (during the credits), it was revealed that Patient C was the sister of one of the KND operatives, due to the fact that Numbuh 74.239 stated guys can't get C.O.O.T.I.E.S. from their sisters when Patient C touched her cowering brother who was hiding behind the wall.
Her clothing and disturbing demeaner, as well as the submarine nature of the place she is imprisioned, make her very similar to the Little Sisters of the BioShock series of video games.
Patient C's design seems to be based on a brunette girl in a light blue dress, one of the background characters in the KND series, except that she has blonde hair and wears a purple dress and a magenta bow.