Mika has short blonde hair kept tied in a bun to the side and is generally seen in her middle school uniform.
Mika is very expressive, reacting strongly at any provocation—she becomes easily angered at her brother and is terrified at the prospect of meeting her crush Hayato (even though she has never met him), becoming overwhelmed at the thought of meeting him and embarrassing herself in front of him.
Mika is introduced as having a crush on Hayato, who she falls in love with after receiving a picture of him from her friend. She plots to meet him and make him like her by showing up at the high school pretending to have come to give her brother the bento he forgot at home, when really she made it with the hope of giving it to Hayato.
She bumps into Kotarou, who wants her bento, and then tries to force Ryuuichi to help her find Hayato after he overhears her plan. Inui arrives, mistakenly thinking she has a crush on Ryuuichi and stops caring after being corrected, saying that she will be disappointed when she meets Hayato. He blames their dad for his dramatic arrival and claims to be helping after Ryuuichi asks if he really is leaving. When Hayato arrives (carrying Taka), she hides behind her brother before Hayato can see her, because she finds him "too hot in person" and is embarrassed. Thinking the bento was made by Saikawa, Taka eagerly tries it, then spits it out and calls it gross, causing Mika to run away. After even Hayato admits it looks terrible, Inui takes the bento claiming it is because she must have made it for him. Inui plans to throw it away but eats it instead after thinking about how if Ryuuichi was her brother, he would have eaten it all instead.
Buried in her pillow after quietly getting through the school day and bursting into tears upon reaching home, Mika hides in her room and refuses to leave for dinner. Still tearful, she heads for the bathroom and starts shouting abuse at her brother after it turns out he is using the toilet. Surprised when he reveals that he ate her bento and it made him sick, she tells him he is gross and not cute at all, causing him to silently declare that he will never do anything nice for her again.