C-ko Kotobuki is A-ko's best friend and has depended on her since they met in kindergarten. C-ko can be considered quite childlike. She is described as "cute" and innocent, tends to be hyperactive, and, in the Japanese version, frequently refers to herself in the third person. C-ko can often be sensitive and immature, and because of this, she has a tendency to cry. Sometimes she can also be selfish and possessive, particularly in regards to A-ko, though whether this is because of unvoiced romantic interest in her friend is unknown. Nevertheless, she is generally a nice, caring, and trusting person, especially towards her friend A-ko, and she usually means well.

In the first installment of the series, C-ko is revealed to be the Fourth Princess of the Fifth Queen of the Lepton Kingdom of Alpha Cygni, an all-female race of extraterrestrial aliens. She crash-landed on Earth when she was an infant, and the Lepton Kingdom has been searching fervently for her ever since. However, C-ko is fairly uninterested in her unearthly ancestry and wants nothing more than to remain on Earth with A-ko.

C-ko enjoys cooking, but the food that she prepares is largely inedible and often contains ingredients not meant for human consumption.

In episode three and four of the main series, we see her having trouble sleeping, usually falling off of the bed after tossing and turning. From episode one to episode three, where we see her face flattened against a toy shop window, indicating that she loves looking at dolls. From episode one and four, we learn she is an avid moviegoer, usually taking A-ko along with her to watch.

The Vs. version of C-ko is identical to her mainstream counterpart in personality, but is only ten years old, the sole daughter and heir to an incredibly wealthy man.

(Source: Wikipedia)