Ichiyō Higuchi is a member of the Port Mafia in Yokohama, serving as the deputy and assistant to Ryūnosuke Akutagawa. Little is revealed about her life before joining the organization, but it is known that she lives with a younger sister. She stands out with her honey-blonde hair, typically tied in a messy bun, and deep brown eyes, often seen wearing a dark pantsuit or a black suit with rectangular blue sunglasses. Osamu Dazai once described her appearance as delicate and lovely as a water lily. Physically, she is notably short, standing at 159 cm.

In her role as a mafioso, Higuchi presents herself as ruthless, professional, and no-nonsense, willing to kill to complete her missions and showing little sympathy for her targets. However, beneath this tough exterior, her personality is considered somewhat ill-suited for the mafia world. She is prone to self-doubt, hesitation, and panic under pressure, which can leave her vulnerable in combat. She is also impulsive and sometimes takes drastic measures that go against direct orders. Despite these insecurities, she possesses a strong resolve and an honest, determined nature. She is deeply committed to the Port Mafia's role in protecting Yokohama and carries out her duties with a sense of responsibility.

Her primary motivation is overwhelmingly tied to her loyalty to Ryūnosuke Akutagawa. She is completely devoted to him, acting as his bodyguard and constantly offering her assistance, which he typically refuses, calling her unnecessary or treating her harshly. Her feelings for him go beyond professional respect, with strong implications of romantic love. She is very protective of him, especially concerning his poor health, and has gone to extreme lengths to rescue him when he was abducted, an act that finally earned her a rare word of thanks from him. Her affection is also evident in her attempts to get Akutagawa's sister, Gin, to call her sister-in-law.

Within the Port Mafia, Higuchi is part of the special assault unit directly under the boss's command, alongside Akutagawa. She holds the authority to command the militant group known as the Black Lizard. Her position is complicated by her lack of a supernatural gift. Unlike most of her powerful colleagues, she does not possess any supernatural ability. Official guidebooks have confirmed this, though earlier material listed her ability as unknown. As a result, she feels an inner frustration and insecurity over her inability to inspire fear or command the same respect as someone like Akutagawa, leading her subordinates to initially follow her only out of fear of his wrath.

To compensate for her lack of an ability, Higuchi is highly proficient with firearms. She is skilled in the use of handguns and submachine guns, often dual-wielding, and is capable of taking down large groups of enemies with her weapons alone. Her combat style relies entirely on this marksmanship and her hand-to-hand fighting skills, making her one of the few individuals in the organization to operate effectively without a supernatural power.

Throughout the story, Higuchi experiences significant development. Initially seen as an ineffective and overeager subordinate who makes blunders, her determination is tested when Akutagawa is incapacitated and later kidnapped. Against orders and despite being warned it is suicide, she launches a one-woman rescue mission. This act of courage and resolve earns her the grudging respect of the Black Lizard, who come to her aid. This arc marks a turning point, proving her worth beyond her fearsome loyalty and demonstrating that her strength lies in her unwavering will. Later, during the Decay of the Angel incident, she is bitten by Akutagawa after he is transformed into a vampire, becoming his thrall and attacking her own allies, including Gin and Hirotsu.