Mikado Mariya♀

Mariya is Mizuho's first cousin and childhood friend who knows him very well. When it was decided that Mizuho would transfer to the all-girls school she also attends, she took it upon herself to transform him into a beautiful girl by use of makeup and seemed to have a lot of fun dressing him up.

Mariya has a very spirited personality which she uses often to make sure Mizuho is acting feminine in every way: appearance, mannerisms, and even speech. This often includes using mild violence. Also, she has a temper, of which she has little control and which sometimes causes her to do unadvisable things or carry grudges to an extreme - such as her grudge against Takako. During most of the time that Mariya has known Mizuho, she has always been there as his support whenever he needed her, though later on in the series she finds that without her even realizing it, he has already managed to stand on his own in terms of self-confidence and maturity. Initially, she is very uneasy about this change and is unsure on how to act towards Mizuho after this revelation. Also, she discovers herself to be harboring some romantic feelings for Mizuho, of which she had been unaware herself. Mariya is in the track and field club and she has another name "Matsurika no Kimi" at Seio Girls' Academy. She also is Yukari's onee-sama.

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[Taken from Wikipedia.]