Bimii is a central figure in Mikagura School Suite, initially appearing as a small, winged, cat-like creature who accompanies the protagonist Eruna Ichinomiya from her very first day at Mikagura Academy. His real name is written with unpronounceable symbols, but because Eruna found his appearance "bimyou" (a Japanese word meaning questionable or weird), she gave him the nickname Bimii, which sticks throughout the series. Despite his unusual form, Bimii is a member of the teaching staff at the academy and is often referred to as "Sensei" by other students. His true nature is eventually revealed when he transforms back into his original human form, that of a middle-school-aged boy with pale pink, messy hair, who is almost always seen chewing pink bubblegum.

In terms of personality, Bimii is a complex character whose demeanor shifts somewhat depending on his form. As a flying cat, he is generally patient and serves as a straight man to Eruna's chaotic and often perverted antics. He is frequently the victim of her pranks and experiments, yet he remains a loyal companion, providing her with essential explanations about the school's battle-based club system and acting as her guide. He has a habit of adding the suffix "ryui" to the end of his sentences. He dislikes being treated as a pet and especially resents when his food is referred to as "feed" or "bait". When he reverts to his human form, his personality shifts to project a cooler, more confident image, though he still retains some of his old verbal tics, which can be a source of frustration for him. Despite the change in appearance, his treatment by Eruna and others remains as irreverent as ever.

Bimii's primary motivation appears to be fulfilling his role as a teacher and overseer. His duty seems to involve following a specific student each year, acting as a referee during club battles, and providing guidance. He had previously trained Seisa Mikagura, the principal's granddaughter, to become the strongest student in the academy, and later takes Eruna under his wing. He is instrumental in helping Eruna navigate the school's rules, eventually even transforming into his human form to serve as the temporary third member required for her to officially establish her own club, the "I'll Figure It Out Later Club". His deep knowledge of the school's history is crucial, as he is the one who reveals the traumatic backstory of Seisa's past betrayal by an upperclassman in the Photography Club, which led to Seisa's emotional withdrawal and the manifestation of her uncontrollable "Killing Art" ability.

Bimii's key relationships are defined by his role as a mentor. His bond with Eruna is the most prominent; while he is often exasperated by her, he is also devoted to her success and well-being, acting as her constant companion and advisor. His history with Seisa is also significant, as he was instrumental in training her, and he seems to hold a position of authority or respect even in relation to powerful students like Kyoma Kuzuryuu, with whom he has a mutual, if not friendly, respect. He also maintains a professional relationship with other students, such as Himi Yasaka, who calls him "Sensei".

Bimii undergoes a notable development, primarily physical, when he reverts to his human form in the fifth novel. This change is accompanied by a slight shift in personality, becoming more serious and cool-headed, though his fundamental nature as a guide and mentor remains. His abilities are significant, befitting his status as a teacher. As a cat, he can fly and is seemingly invisible to normal people who cannot see the supernatural aspects of the academy. In combat, he is indicated to be extremely strong, having successfully trained Seisa to be the academy's strongest and later being confident enough to challenge Otone Fujishiro without using his ability, a challenge that resulted in Otone's concession. He also has the ability to transform between his cat and human forms.