Shiki Tōno is a high school student plagued by anemia, causing frequent fainting spells and deep fatigue. His unruly black hair frames rectangular glasses essential for suppressing his Mystic Eyes of Death Perception. This power, granting sight and interaction with the "lines of death" inherent to all things, stems from a near-fatal childhood accident that scarred his chest and fragmented his memories. Biologically descended from the demon-slaying Nanaya assassins, he was adopted by the Tōno family after their destruction. This heritage instills dormant combat and assassination instincts that awaken under extreme pressure.

Personality-wise, he greets the world with playful teasing, especially directed at his sister Akiha Tōno and the maids Hisui and Kohaku. Despite facing mortality, he cherishes the mundane. His comedic portrayal highlights a lighthearted approach to romance, contrasting with more serious characters. He navigates relationships with multiple female characters like Arcueid Brunestud, Ciel, and Akiha, often becoming the central prize in their humorous rivalries. This forces him into constant scheduling conflicts when juggling group activities or dates.

Key recurring elements involve his battles with overwhelming drowsiness, requiring others to wake him, and his reliance on glasses to manage the strain of his Mystic Eyes. His anemia often triggers abrupt, comedic collapses. Specific storylines see him caring for a temporarily regressed Akiha after she drinks altered tea, handling her childlike behavior with flustered patience. Another plot involves Kohaku teleporting him experimentally, sparking a frantic effort to keep simultaneous commitments with Arcueid, Ciel, and Hisui.

His interactions showcase adaptability to the absurd, whether participating in unconventional Holy Grail Wars or cafe events. Relationships retain core traits from their origins—like Arcueid’s public affection causing him embarrassment—while amplifying comedic potential through exaggerated reactions and timing conflicts.