Shinya is the alternate personality of Mahiru Banba, emerging from severe childhood trauma. Mahiru endured prolonged captivity and abuse by her uncle, who imprisoned her in a dark cave, subjected her to constant photography, and pressured her to become his bride. This trauma left Mahiru with intense phobias of darkness and bright light. Shinya manifested as an aggressive, confident defense mechanism to protect Mahiru, taking control of their shared body at night while Mahiru rests dormant. The scar on the left side of their face originated when Shinya killed their uncle during an escape attempt.

Personality-wise, Shinya contrasts Mahiru's timidity with loud assertiveness and ruthless behavior, often displaying a psychotic grin. Despite her violence, she harbors deep concern for Mahiru's well-being and desires a normal life for her. However, Shinya willingly commits murder to obtain relics—items taken from the deceased—fulfilling Mahiru's unspoken wish to fill an emotional void. Bright light triggers physical distress and potential mental breakdowns due to Mahiru's trauma. Shinya wields a heavy sledgehammer, leveraging temporary, adrenaline-fueled superhuman strength that strains their body. Her hobbies include crossword puzzles and she favors dried squid.

Within Myojo Academy's Class Black, Shinya operates under Mahiru's identity as Student No. 12, one of twelve assassins targeting Haru Ichinose. Lacking a personal wish, she participates to obtain a specific relic by killing Haru: the phone strap Mahiru had given her. Shinya executes nighttime assassination attempts, ambushing Haru and Tokaku Azuma with her sledgehammer. She forms a temporary alliance with Isuke Inukai, who betrays and incapacitates her during a joint attack, leading to their expulsion from Class Black.

Both Mahiru and Shinya survive the events of Class Black. Post-expulsion, Mahiru is seen working as a maid for Sumireko Hanabusa. While Shinya's existence persists, her active role diminishes significantly after the Class Black incident.