Aizen Kunitoshi is a tantou, or short sword, whose human form manifests the spirit of a blade forged by the second generation master of the Rai school, a smith commonly known as Niji Kunitoshi. His most distinctive physical feature is the carving of Aizen Myou'ou, a Buddhist Wisdom King, on his blade, which is a source of great personal pride. He often points to this engraving and asks others if they think it looks cool, indicating that he equates this sacred symbol with his own sense of style and strength.

In terms of personality, Aizen Kunitoshi is exceptionally energetic, boisterous, and always seeking excitement. He constantly looks for opportunities to celebrate and has an overwhelming love for festivals, often shouting phrases like "Festival, festival!" when a battle or a new mission begins. This exuberance makes him restless in quiet or peaceful situations; he has openly admitted that he cannot sit still when it is too calm. His approach to fighting is as loud and straightforward as his personality. He prefers to charge into battle as the vanguard and dismisses complex strategies in favor of improvising on the spot. He sees combat less as a solemn duty and more as a thrilling brawl or a performance where he can stand out and show off.

His motivations are closely tied to this desire for recognition and fun. He wants to be flashy and noticed, declaring his intention to "stand out blindingly" when serving as a team captain. He is driven by the promise of action and celebration, and he shows visible disappointment when assigned to a long expedition that does not involve either fighting or partying. Despite his impulsive and carefree demeanor, his dialogue reveals a competitive edge, as he claims he will not lose even to a tachi, a long sword, implying a desire to prove that his smaller stature and lively spirit are formidable assets in a fight.

Within the story of Touken Ranbu: Hanamaru, Aizen Kunitoshi occupies the role of a spirited and slightly unruly younger member of the citadel. His key relationships are primarily defined by his connection to other swords of the Rai school. He specifically mentions Kuniyuki, referring to him as the group's guardian or caretaker, which suggests a family-like dynamic where Kuniyuki is the responsible older figure managing Aizen's boundless energy. He also enthusiastically greets new swords forged at the citadel, showing a friendly and welcoming nature that helps integrate new characters into the group.

Throughout the series, Aizen Kunitoshi shows development primarily through his potential for growth as a warrior. In his standard form, he is a tantou with specific combat parameters, but he is capable of undergoing a special training process known as Kiwame. In this enhanced form, his combat abilities, particularly his mobility and scouting capabilities, increase dramatically, reflecting a maturation of his skills while retaining his fundamental energetic nature. His notable abilities as a tantou include high mobility and scouting potential, especially after Kiwame training, which allows him to move quickly on the battlefield and gather intelligence for his team. His fighting style is characterized by a single-minded focus on aggressive, flashy attacks, as demonstrated by his battle cry to send enemies flying, and he carries the conceptual "protection" of the Aizen Myou'ou into combat with him.