Shiki commands a central role through his imposing presence and formidable combat prowess. He possesses black hair, crimson eyes, pale skin, and stands approximately 188 cm tall. His attire is consistently black, accented by silver accessories: bracelets, a studded belt, and cross necklaces. He constantly carries a katana, wielding it with exceptional skill and respect. His physical abilities include heightened speed, ground-shattering attacks, and masterful swordsmanship, establishing him among Toshima's strongest fighters. Consuming undiluted Line from Nano's blood intensifies his power, enabling him to effortlessly overpower adversaries like the Executioners.

His background involves the criminal underworld and military contract work. A pivotal moment occurred when he returned from a mission to find his base destroyed by ENED's enhanced soldiers, including Nicole Premier. Locking eyes with Nano instilled in Shiki his first experience of fear. Though he fought through five soldiers to attack Nano, he was spared—an act fueling his lifelong obsession to surpass and kill Nano. This drive led him to co-found the drug ring Vischio and the lethal game Igura with Arbitro, using them to encounter Line-compatible opponents and gather strength for his vendetta. Shiki harbors no interest in Vischio's profits, solely focused on his goal.

Shiki exhibits a sadistic, arrogant, and violent personality, viewing Igura participants as "trash" and deriving pleasure from killing them. He expresses disdain for those reliant on others or the drug Line. His interactions are marked by brutality, including threats of rape, physical torture, and psychological domination. Rare glimpses of complexity emerge, such as melancholy after killing his half-brother Rin or fascination with those who defy him, like Akira. His single-minded pursuit of Nano defines his actions, often overriding all other considerations.

Relationships are defined by conflict and manipulation. Rin, his younger half-brother, became a target after Shiki betrayed and murdered Rin's team, Pesca Corsica. Shiki kills Rin in his own route, displaying ambiguous remorse afterward. With Akira, Shiki evolves from dismissive ("small fry") to intensely fixated, captivated by Akira's refusal to submit even when outmatched. This leads Shiki to abduct Akira, subjecting him to days of physical, sexual, and psychological torment, including forced piercings and rape. In Shiki's route, Akira develops Stockholm syndrome, remaining with him despite opportunities to escape. Shiki holds a partnership with Arbitro but despises his greed and sadistic hobbies. Nano remains his ultimate nemesis, embodying Shiki's fear and driving his every action.

His story arc varies across media. In the anime, he repeatedly encounters Akira, sparing him multiple times due to intrigue over his defiant demeanor. During the civil war climax, Shiki consumes Nano's blood, gains augmented abilities, and battles Nano. He sustains a severe diagonal torso wound from Nano but survives to confront Akira one final time amidst Arbitro's new Line supply, leaving their clash unresolved.

In the game, outcomes diverge based on routes:
- His route sees him forcing Akira into captivity after Keisuke's death. After killing Rin and battling Nano, Shiki either rejects Nano's blood, kills him, and becomes catatonic (Good Ending); consumes the blood and descends into madness as a tyrant (Bad Ending 1); or gains controlled power through Akira's blood and establishes a military dictatorship (Bad Ending 2).
- Rin's route culminates in Shiki being wounded and defeated by Rin in a final duel.
- Nano's route implies Shiki's death at Nano's hands, with Nano returning bloodied and carrying Shiki's katana.