Ichiyou Higuchi is a member of the Port Mafia, a powerful criminal organization, where she serves as a subordinate and self-appointed bodyguard to Ryuunosuke Akutagawa. She is also the commanding officer of the organization's armed militant group, the Black Lizard, though she holds this authority as a deputy under Akutagawa. Her birthday is May 2.

Physically, Higuchi is a young woman of relatively short stature, with honey-blonde hair that she typically keeps in a messy bun, and deep brown eyes. Her standard attire is a professional black pantsuit, usually left unbuttoned, and she is often seen wearing rectangular blue sunglasses. When working undercover, she adopts a more polished look, buttoning her jacket neatly and forgoing her sunglasses.

Higuchi's personality is defined by a sharp contrast between ruthless efficiency on the job and deep-seated personal insecurities. As a Port Mafia operative, she is loyal, bloodthirsty, and has no qualms about using lethal force to carry out her orders, sometimes displaying a reckless disregard for capturing targets alive. However, she is also easily shaken by her own mistakes, and this self-doubt can hinder her performance in combat.

Her most prominent trait is her extreme, one-sided loyalty to Akutagawa. She is obsessively devoted to him, enduring his verbal and physical mistreatment, including his frequent declarations that she is unnecessary, while risking her life to protect him. This devotion is heavily implied to be romantic, as she tries to convince Akutagawa's younger sister, Gin, to call her sister-in-law. Despite her fierce exterior, Higuchi harbors inner frustration and insecurity over her place in the mafia, largely because she lacks a supernatural ability that would instill fear in subordinates like Akutagawa does.

The details of Higuchi's past and why she joined the Port Mafia are not explored in depth, but it is known that she lives with her younger sister. In her first appearance, she infiltrated the Armed Detective Agency by posing as a client, successfully leading its members into a trap. The mission ultimately failed, and she responded by ordering the Black Lizard to raid the Agency, which also ended in failure.

A significant turning point for her character occurs when Akutagawa is severely wounded and subsequently abducted by a rival organization. With Akutagawa in a coma, Higuchi assumed command of the Black Lizard. However, the unit's leaders, who only followed Akutagawa out of fear and respect, made it clear that Higuchi had not yet earned that same respect, and some even suggested it would be safer for her to leave the mafia. Ignoring orders and knowing it was a suicide mission, Higuchi launched a rogue rescue operation. Though she was overpowered and wounded, her sheer determination impressed the Black Lizard, who arrived to save her and helped secure Akutagawa. In a rare moment, Akutagawa thanked her for her efforts.

During the Decay of the Angel incident, Higuchi was bitten by a vampire version of Akutagawa, transforming her into his vampire subordinate. In this state, she turned on her allies, attacking Gin Akutagawa and Ryurou Hirotsu. Outside of combat, Higuchi displays a lighter, more awkward side, such as when she retaliated against enemies by mailing them excessive cash-on-delivery packages.

Higuchi does not possess any supernatural ability, a fact confirmed by official guidebooks despite early ambiguous listings in promotional materials. To compensate, she is highly proficient with firearms, demonstrating mastery with pistols and submachine guns, often dual-wielding them to take out large groups of enemies. While skilled, her lack of an ability means she is usually outmatched against superpowered foes and often requires support to prevail in a direct confrontation.