One of the rabbits Haru was taking care of. 

Following the rooftop incident, she ended up possessing Mikami and becoming her body's dominant split personality - an extremely carefree, friendly and upbeat girl that always speaks her mind. She enjoys showering Haru with over-the-top displays of affection. Due to being a rabbit, she doesn't understand a fair amount of things humans do, resulting in a wide plethora of lewd, awkward and humorous situations.

She's capable of manipulating others' memories via signals she sends from her ears, allowing her to blend seamlessly into the school's environment and live with Haru's family by convincing everybody around her that she's always been there.

Mikami Inaba was fighting a person from another school on her school's rooftop. As they fought, her friend Haru appeared. Seeing Mikami distracted, the man charged at her, causing both of them to break the nearby rabbit cages &amp; fall off the roof. As Mikami fell, she desperately grabbed the rabbit. Suddenly, a flash or pink light appeared. Mikami fused with the rabbit, and gained another personality, called Mimika. Now, Mikami only appears when various people threaten her and her friends. Otherwise, she takes on the form of Mimika.
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(Source: Anime News Network)

Mikami Inaba was fighting a person from another school on her school's rooftop. As they fought, her friend Haru appeared. Seeing Mikami distracted, the man charged at her, causing both of them to break the nearby rabbit cages & fall off the roof. As Mikami fell, she desperately grabbed the rabbit. Suddenly, a flash or pink light appeared. Mikami fused with the rabbit, and gained another personality, called Mimika. Now, Mikami only appears when various people threaten her and her friends. Otherwise, she takes on the form of Mimika.
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(Source: ANN)