Akiha Tohno is the younger sister of the protagonist, Shiki, and serves as the head of the prestigious Tohno household following the death of her father. Her background is defined by a sheltered childhood within the family mansion, where she had little freedom to play outside. Her early years were marked by a complex relationship with her biological brother, SHIKI, whose behavior she found strangely different and disturbing. When Shiki was adopted into the family, she initially disliked him due to his sharing a name with her brother. However, her feelings changed when he invited her to play, giving her a rare experience of childhood joy. This period of happiness ended with a terrible accident eight years before the main story. During that incident, SHIKI lost control of his demonic blood and attacked Akiha. Shiki protected her, suffering a fatal wound. To save his life, Akiha unconsciously used the power of her own non-human blood to give him half of her life force. This event caused her own demonic heritage to fully awaken. Following this, her biological brother was confined, and Akiha severed all ties with him, thereafter recognizing only Shiki as her true brother. In the years that followed, her father sent Shiki away to live with another family, and Akiha was placed in a boarding school, making contact between them nearly impossible. After her father's death, Akiha became the head of the Tohno family and, overruling the protests of her relatives, called Shiki back to live in the family mansion.

Akiha's personality is a blend of strict formality and deeply concealed care. As the head of a noble family, she carries herself with flawless etiquette and a serious, dignified bearing. She is sharp-tongued, strict, and has a reputation for being difficult to approach, often speaking in a commanding and critical manner. This is especially true in her interactions with Shiki, whom she constantly scolds for his lifestyle, his late nights, and his manners. Her desire is to reform him into a proper human being. This strict exterior, however, hides a profoundly protective and caring nature. She is intensely jealous of other girls who spend time with Shiki and is determined to know everything about his day. Her constant supervision and suspicion of his nighttime activities stem from genuine worry for his well-being. She is incredibly shy about her true feelings, often acting in complete contradiction to them, but deep down, she called Shiki back to the mansion because she truly loves him and wants to be near him, making him the most important person in the world.

Akiha's primary motivation is to protect Shiki and uphold the Tohno family name. Her role in the story is central, as her decision to bring Shiki back to the mansion sets the entire narrative in motion. She acts as the stringent head of the household, a figure of authority who attempts to control Shiki's actions and maintain order. As a result of the accident that saved Shiki's life, Akiha now requires twice the amount of life force, or heat, of an average person because she is sustaining both her own life and his. To compensate for this deficiency, she must absorb heat from her surroundings and from other living creatures. The most efficient method for her to do this is by drinking fresh blood, which she does nightly from the maid, Kohaku, to keep her demonic nature at bay.

Key relationships define much of her character. Her bond with Shiki is the most significant, marked by her strict oversight and unspoken deep affection. Her relationship with her biological brother, SHIKI, is completely severed following his inversion and attack, and she refuses to acknowledge him as her brother. With the maids, Kohaku and Hisui, Akiha shares a complex dynamic. She is the master of the house, but she also intervened to free Kohaku from their father's abuse. Despite this, Kohaku secretly provides her with blood each night, a situation that continues to bind them together in a complicated arrangement. Akiha also holds a position of high authority and respect at her school, Asagami Private Girls Academy, where she serves as the student council vice-president.

Akiha undergoes subtle but important development, primarily concerning her control over her non-human heritage. She possesses powerful abilities as a half-demon or half-oni, a "Mixed-Blood." Her most notable power is called Caging Hair, or Plunder. This ability allows her to plunder the heat, or lifeforce, from any living creature within her line of sight. To those with spiritual perception, this power manifests as crimson red strands of hair that wrap around the target and drain its heat, creating a powerful burning sensation. While it appears as though she is setting things on fire, she is actually draining life force, and the effect stops the moment she ceases. This power is absolute against normal people but is less effective against beings of much higher spiritual status. When her demonic blood advances too far, she enters a state known as Crimson Red Vermillion. In this form, her long black hair turns a vivid red, and her power increases tremendously. While most individuals lose their sanity in this state, Akiha's superior quality as a half-blood allows her to retain a degree of self-control. This form is a manifestation of her Tohno lineage, which is said to have connections to ancient oni and a figure named Momiji, whose name shares thematic ties to Akiha's own. Her power is a double-edged sword, as the constant strain of suppressing her demonic side and supplying Shiki with life force causes her intense physical pain, a burden she bears bravely for his sake.