Masaki Ōsoto, a medical student, arrived at the Tasokare Hotel after nearly dying from impalement on a pointed fence post during a rain-soaked accident. This occurred en route to assaulting one of his girlfriends. Ōsoto's past was defined by severe parental neglect. Born to highly accomplished, demanding parents, he relentlessly struggled to meet their expectations yet consistently failed to earn their approval. This persistent rejection eroded his self-esteem, leading him to kill his pet bird, Pi-chan, to reclaim a sense of control. This act escalated into harming larger animals and eventually humans, starting with a homeless person. He meticulously documented his victims on a five-star scale based on the satisfaction their deaths provided. Despite maintaining perfect grades as a medical student, his parents remained unimpressed, instead expressing admiration for hotel concierge Haruto Atori—whom they wished was their son. This fueled Ōsoto's obsessive jealousy, prompting him to stalk Atori before ultimately pushing him in front of a train, causing Atori's death. After this murder, Ōsoto found no satisfaction in subsequent crimes but continued them in a futile attempt to feel anything.

Initially at the hotel, Ōsoto suffered amnesia and lacked physical facial features, his head appearing as a purple pansy. Assigned to Neko Tsukahara's care, he gradually recovered clues about his identity. When Neko discovered his victim records, he regained full memory but falsely claimed to be a detective to conceal his crimes. His facade partially crumbled after Neko deduced his near-fatal accident, causing his impalement wound to manifest and bleed. Following a failed attempt to take Ruri hostage during interrogation, the Manager intervened, revealing that killing within the hotel would condemn him to Hell. Learning that staff and guests typically forget their hotel experiences upon returning to life, Ōsoto pretended ignorance about his survival to prolong his stay, citing a desire to remain near Atori. Though distrusted by staff, the Manager permitted his continued residence conditional on cooperation.

During his hotel stay, Ōsoto displayed a complex blend of manipulation and genuine engagement. He frequently assisted Neko with investigations and puzzles, developing a rapport bordering on friendship. He expressed fascination with her deductive abilities, comparing their dynamic to Sherlock Holmes and Moriarty. His underlying motive, however, centered on assimilating Atori—driven by a belief that consuming another would absorb their essence. He prepared a hidden basement in his room, stocked with torture equipment and photographs of Atori, orchestrating a plan to lure Atori during a staff shift change. When Neko intervened, he confronted her outside the hotel. She deduced his envy-driven motives and intent to "eat" Atori. After Neko hesitated to shoot him with the Manager's gun, Ōsoto overpowered her. Atori intervened and accidentally shot Ōsoto, killing him and triggering both their descents into Hell through a portal.

In the true ending, Neko used a time-travel mechanism to prevent Atori's murder. She confronted Ōsoto at the train station before his original attack, opening the gate of Hell to drag him in. Though he pulled her in during the fall, she escaped using her umbrella. An alternate conclusion shows Neko becoming Ōsoto's accomplice in the real world, aiding his crimes in exchange for sparing Atori, leaving the gate of Hell sealed.

Ōsoto's personality combined intelligence, eloquence, and superficial charm with profound psychological instability. He maintained a polite, laid-back demeanor but exhibited chilling condescension when threatened. His fashion reflected duality: refined Victorian-style attire resembling Sherlock Holmes in the hotel, contrasting with casual, unassuming real-world clothing. His elegant, book-filled hotel room concealed a bloodstained basement symbolizing his hidden brutality.