Mikako is an upperclassman to Tomoki and Mitsuki and the President of the Student Council. She is Sugata's childhood friend, whom she refers to as "Ei-kun", and the heiress of a Yakuza family. She is manipulative, cunning, highly intelligent and somewhat sadistic, usually towards Tomoki. She is forced to take supplementary lessons due to "failing in morals". She also is equal to Sugata in terms of combat abilities. However, Nymph states that she has a grip power of 400kg, which serves to indicate she has a great degree of strength, especially for a normal human.
She is the chief mastermind of many schemes in the manga, and Sugata normally enlists the help of Tomoki (and occasionally some others) by persuading him to totally shut down her plans. Many of her schemes are directed to Tomoki, serving to put him in undesirable situations and humiliating him, which suggests she sees him as a toy for her personal entertainment.
Nevertheless, in situations of genuine emotional distress, she can be surprisingly thoughtful (if in her own unique way); when Tomoki breaks the contract between him and Ikaros in one chapter, Mikako immediately repairs their relationship and contract by ordering Ikaros to throw Tomoki into the ocean.