Hōkōin emerges as a primary antagonist within the series, firmly aligned with the Shinsegumi and their chaotic warmongering alongside Hitotsubashi Shigeyoshi. Her origins remain shrouded in mystery, defined solely by her dual roles as an assassin and a practitioner of sexual manipulation—a self-styled "sex-witch" who preys on targets including individuals as young as seventeen. She exploits sexual activities with chilling precision while possessing unspoken mastery of kung fu, though she seldom engages directly in combat.

Visually arresting, Hōkōin cuts a voluptuous figure with flowing black hair, piercing aqua eyes, and an intricately styled bun. Her wardrobe favors provocative ensembles: a revealing purple qípáo baring half her body, purple pumps, and flame-like markings adorning her legs—whether paint or tattoos remains ambiguous. She accentuates her presence with elongated fingernail tips and occasionally dons traditional Chinese-inspired attire like a blue hànfú or a floral-patterned red wafuku paired with a head scarf. Her appearance shifts subtly across episodes, with variations in hairstyle and leg art.

Her personality drips with overt eroticism, echoing the cadence of an oiran while lacing dialogue with sexual provocations. A sadistic and remorseless predator, she relishes tormenting victims, oscillating between arousal and boredom when situations lack amusement—exemplified when she dismissed assassinating Keiichiro Tokugawa as tedious. Ruthlessly calculating and resourceful, she embodies the Shinsegumi’s merciless ethos. Her alliances include Shigeyoshi Hitotsubashi as her principal, albeit enigmatic, collaborator. Tsubaki serves as her assistant, enduring non-consensual sexual advances. She relentlessly targets Keiichiro Tokugawa, tasked with eliminating him before he masters the Onigami Susanoo, and torments Kiriko Hattori through harassment and abduction.

In battle, Hōkōin commands the supernatural mecha Hokoinkugutsu, wielding a naginata with lethal grace. She demonstrates superhuman speed and strength, once felling multiple foes via wind gusts from her colossal iron fan. Her teleportation ability—marked by pink smoke and violent winds—aids evasion and kidnappings, as with Kiriko.

Her narrative thrust revolves around Shigeyoshi’s orders. Early missions see her coldly executing a pleading man and stabbing Toramaru, who survives. She ambushes Keiichiro and Kiriko, overwhelming them with her Onigami. Later, she provokes Keiichiro into activating Susanoo, mocking his "weakness" and "virginity" until his transformation forces her retreat. She later confronts Susanoo at the Cherry Blossom Baths, exhilarated yet fleeing after one strike. Throughout, she monitors Keiichiro’s growth and manipulates Kiriko’s insecurities about piloting Susanoo, her motives deliberately opaque. Her actions fuel conflict without personal evolution or backstory.

Her predatory philosophy echoes in quotes like "Love is only true when you don’t want anything in return" and "Young fruit tastes best just before it falls to the ground."