OVA
Description
Mika Harima, born into wealth, attended middle school with Anri Sonohara. She initially befriended Anri to boost her social standing, yet developed authentic concern for her. Her life pivoted dramatically when Seiji Yagiri rescued them from thugs, igniting an obsessive infatuation. Mika stalked Seiji, meticulously gathering information before breaking into his apartment. There, she discovered Celty Sturluson's preserved head, provoking Seiji into a murderous rage where he slammed her head against a wall.

Mika survived, awakening on an operating table under Namie Yagiri's supervision. Namie offered plastic surgery to reshape Mika's appearance to match Celty's head, combined with memory-altering drugs to trick Seiji. Mika consented to the surgery, resulting in lighter brown hair, grey-green eyes, paler skin, and a distinct zigzag neck scar. She learned the head belonged to Celty from Shinra Kishitani and was instructed to use that name with Seiji. However, Mika refused the drugs and escaped the lab.

Returning to Raira Academy with her altered appearance, Mika reconnected with Anri while posing as a stranger. Mistaking her for Celty's reanimated head, Seiji accepted her as his girlfriend, calling her "Celty." Mika maintained this deception while covertly receiving threats and monitoring Seiji's communications. She destroyed his phone to sever ties with Namie, triggering events where Celty pursued her upon recognizing her face. Fleeing, Mika encountered Mikado Ryuugamine, who provided brief shelter. Warned of imminent danger by Izaya Orihara, she fled again, leading to an intervention by Kyouhei Kadota's group.

During the Dollars meeting, Seiji attacked Celty to reclaim Mika, believing her to be the head. Mika intervened, revealing her true identity, recounting her surgery and enduring love for Seiji despite his violence. Her confession led Seiji to accept her as his girlfriend, conditionally—until he found Celty's real head. They later joined the Dollars, with Mika frequently seen alongside Seiji at school. She continued offering cautious advice to Anri, particularly during Yellow Scarves conflicts.

Mika demonstrated advanced technical skills, wiretapping homes and phones of individuals connected to Seiji, including Mikado, Anri, and Izaya. She amassed extensive intelligence on Ikebukuro's underground but primarily used it to shield Seiji, aware he might intervene recklessly out of guilt. When Namie threatened to reverse her surgery, Mika blackmailed her with evidence of Namie and Izaya's illicit activities, vowing to protect her relationship at all costs. She also expressed a desire to consume Celty's head to permanently secure Seiji's affection.

In later arcs, Mika's possessiveness and protective instincts persisted. She alerted Anri to dangers from the Tandeki Group and experienced traumatic visions, including fainting after imagining a needle forced into her eye. After Namie's death, she disclosed the truth to Seiji, causing temporary estrangement. When Seiji exhibited strange behavior—avoiding light and staring blankly—Mika sought help, ultimately aiding in locating him during an exorcism ritual. Throughout, she remained desperate to keep Seiji by her side despite repeated turmoil.

Mika is characterized by hyperactive, cheerful energy and obsessive devotion. While her stalking tendencies are evident, Mikado Ryuugamine noted her actions stemmed from genuine love rather than selfishness. She excels in cooking, scoring perfectly in a light novel skill assessment, and utilizes her technical expertise for surveillance. Her complex dynamic with Seiji involves mutual but distorted dependence: he treats her affectionately as a substitute for Celty's head, while she manipulates circumstances to ensure his safety and proximity.