Aiha is a regalia, a spirit who serves the god of war Bishamonten. She has been a member of Bishamon's large family of regalia for fifty years, where her primary role was to serve as her master's main defensive armor. In this capacity, Aiha would transform into a full suit of western-style plate armor, characterized by a long skirt that Bishamon would occasionally step on during battle. In her human form, Aiha is depicted as a young girl of short stature, typically with her light pink hair tied up with a ribbon and dressed in a smart suit. Her name is said to incorporate the meanings of the color indigo and the Tomoe symbol.

Initially, Aiha was a confident and proud regalia, taking great pride in her position as Bishamon's foremost protector. However, her sense of self-worth became deeply shaken when another regalia named Tsuguha was chosen to take her place as Bishamon's primary godly clothing. This change left Aiha feeling neglected, replaced, and starved for the attention and affection of her master. This insecurity festered into intense jealousy, causing her to act out by bullying Tsuguha, including cutting her hair. Her desire to be loved by Bishamon made her vulnerable to the manipulations of Kugaha, another of Bishamon's regalia who was secretly plotting to kill the goddess and force her reincarnation. Consumed by her distress, Aiha accepted medicine from Kugaha that was designed to hide her emotional pain, but which in fact masked a growing blight she was inflicting on Bishamon.

Aiha plays a significant role as an unwitting pawn in Kugaha's scheme during the conflict that drives much of the story. Under the influence of his medicine and her own resentment, she assists Kugaha by using a staff imbued with dark magic to attack the god Yato and helps capture the human girl Hiyori Iki. She believes these actions will help her regain Bishamon's favor. However, when Kugaha's true, coldly ambitious plan is revealed, Aiha is overcome with guilt and horror at what she has done. In a pivotal act of redemption, she defies Kugaha, frees Hiyori and another regalia named Kazuma, and urges them to save Bishamonten. Her confession and bravery help expose the conspiracy. Following these events, Aiha endures a painful ablution ritual to cleanse herself of her blight and prevent her from turning into a monster. She reconciles with Bishamon, who chooses to keep her in the family, and manages to repair her relationship with Tsuguha, with whom she becomes close friends.

As a regalia, Aiha's notable ability is her transformation into a piece of defensive equipment. In her vessel form, she becomes the suit of armor described above, offering Bishamonten extremely high defensive power. Beyond her combat role, Aiha is revealed to have a creative and somewhat romantic side. She enjoys drawing and is particularly fond of creating shoujo manga-style stories, often featuring her master Bishamon in humorous or romantic scenarios, such as with the god Yato.