Akiha is the caring younger sister of Shiki Tohno in Lunar Legend Tsukihime. She has a bit of a crush on her brother and as a result does not get along well with Arcueid Brunestud and Ciel.
Akiha is a member of the Tohno family, a clan with demonic blood. Living under her father Makihisa's strict supervision, Akiha was rarely allowed to leave the mansion to play with others and was subconsciously afraid of her brother, SHIKI, due to feeling something sinister with him. When Shiki moved into the Tohno residence, Akiha hated him at first for having the same name as her brother. However one day while awaiting her tutor, Shiki is able to sneak her outside so she can play with SHIKI and Hisui, which Akiha enjoyed as she finally got some joy of experiencing a normal childhood.
Unfortunately, this happiness didn't last long as SHIKI would fatally wound Shiki when he succumbs to rage from his demonic blood with Akiha as a witness during a day the three were playing together. To save Shiki's life, Akiha would use a portion of her life force to revive him. Shortly after these events, Shiki was forced to move out of the Tohno residence and live with another branch of the family called the Arimas and strict pressure from family and relatives prevented Akiha from having any opportunity to see Shiki. This was bad enough where Akiha was placed at a boarding school for girls and letters she attempted to send Shiki were never answered due to the boy being forbidden from making any contact with her.
Years later, Akiha and Hisui would learn of Kohaku being regularly raped by Makihisa and have her become a maid at Akiha's mansion to free her from the man. Following Makihisa's death, Akiha would become the head of the Tohno household and call Shiki to return to her despite protests from the remaining family members.
Akiha stands at 160 cm (about 63 inches) tall with her three sizes being 73 cm for bust, 57 cm for waist and 79 cm for hips. She has long black hair that extends down to her waist (red when triggering her demon blood) and blue eyes. She has a white hairband in her hair and is seen wearing a white shirt with a red ribbon on the necktie and a long red skirt for casual clothes.
Akiha is very protective of Shiki to the point where she must regularly know of his whereabouts and is easily jealous when he is around other girls. Despite her peevish behavior, she is a caring person at heart and deeply loves Shiki. Because Shiki was hypnotized into thinking he was part of the Tohno family, Akiha plays to the illusion that she is Shiki's younger sister because of feeling glad he is back in her life and wanting to be with him, even at the risk of isolating herself from other members of her family. She is very self-conscious of her flat chest.
Akiha mostly serves in a supporting role during Arcueid and Ciel's routes for the Tsukihime visual novel. In Arcueid's route, Akiha is peevish yet caring and tries to conceal the truth of Shiki's past to him until Roa (occupying SHIKI's body) appears to attack Shiki.
Towards later in the route, Akiha starts spending more time with Shiki by becoming a student at his school. A recent string of murders leads Shiki to later suspect that Akiha may be involved with them until he encounter SHIKI at the family mansion with an unconscious Akiha. SHIKI reveals that he has come to take Akiha "back with him" and reveals their past together. She would eventually awaken and protect Shiki. Upon hearing from Akiha that she no longer acknowledges him as her brother, an enraged SHIKI goes in to kill Shiki until Akiha steps in to take the attack. This event triggers Shiki's Nanaya blood leading him to cut SHIKI's lines of death with the Mystic Eyes of Death Perception.
Afterward, Shiki and Akiha would talk in the latter's room where she reveals the true events of his past. As Akiha starts having spasms and her hair turns red, she reveals that she was aware of not being related to Shiki yet retained the facade because she wanted to be with him. Shiki acknowledges that he sees Akiha as both his sister and a woman, leading the two to kiss and have sex. Following the events, SHIKI would arrive in a jealous rage and abduct Akiha.
Later arriving in the hidden room where SHIKI had been imprisoned, Shiki discovers that SHIKI had turned Akiha into a mindless monster and apparently raped her. The two would fight, with Shiki getting the upper hand yet hesitant to kill his former friend. SHIKI would vanish and return to Akiha's location where he is killed by her after she rips his heart out. You are then given three choices for how to deal with Akiha.
If you choose to kill her (which Akiha wanted in case she became a monster), this triggers a bad end.
If you spare her, this trigger's Akiha's "Normal End". Here, Shiki would regularly visit the monstrous Akiha in the hidden room to allow her to painfully suck his blood to contain her bloodlust, hoping that she will one day be herself again.
If you choose to kill yourself, this triggers Akiha's "True End". With Shiki's death, Akiha regains the portion of her life force she gave to him which restores her sanity and gives her the strength to control her demonic blood. Later returning to where he died, Akiha sees Shiki's knife and touches it. Sensing a faint heart beat from deep inside the knife, she sees this as a sign that Shiki is still alive and holds onto the hope that he will someday return.
Here, Akiha is aware of Kohaku's plans of revenge on her and the Tohno family for her tragic past. Feeling that her family wronged Kohaku, Akiha wished to give Kohaku a chance.
In Hisui's "True End", Akiha is fatally wounded when she uses herself as a shield to protect Kohaku from SHIKI. The act devastates Kohaku emotionally enough where she is driven to kill herself. In Hisui's "Good End", she only has her arm injured and survives.
Here, Akiha hunted down and killed SHIKI to prevent Shiki from having any later encounter with one another. However, SHIKI plants a curse on her before his death where he transfers his madness into her. This leads Akiha to start succumbing to her demon blood where even synchronizing with Kohaku is not enough to control her bloodlust. This leads her to hunt for victims at night to satisfy her bloodlust, though she retains enough of her sanity to prevent killing them.
This madness also drives Akiha into becoming single mindedly obsessed with Shiki, enough to the point where she threatens to take back the life force she gave to him unless he declares that he loves her. Shiki admits that he had returned to the mansion to see her again, yet only seen Akiha as a sister and had also returned to make a promise to Kohaku. This declaration worsens Akiha's condition as she threatens to kill Shiki.
In Kohaku's "True End", Shiki synchronizes with Kohaku to gain strength to fight Akiha. When he is able to get the upper hand in the fight, Shiki finds himself incapable of killing Akiha. This act, combined with Kohaku's earlier declaration to end her plans of revenge on the Tohno family, is enough to restore Akiha's sanity. While still able to live a normal life, Akiha still finds herself having the desire to drink blood, which is resolved by having her drink blood packs meant to be used for transfusions.
With her demon blood, Akiha possesses an ability called the Caging Hair where her hair turns red and is able to control its movement to attack and absorb the heat (life force) of any foe within the range of her vision. She can extend the length of her hair and passively absorb the heat of anyone that comes into contact with it. The amount of time that the Caging Hair stays active depends on how much heat Akiha absorbs. At its maximum output, Akiha is capable of covering an entire area with her hair thus preventing any type of escape for her foe. She is also capable of transferring her life force to others, as she did to save Shiki's life years ago when he was fatally wounded by SHIKI. The absorption abilities of the hair are ineffective against foes with a high level of spiritual defense such as Arcueid and any Heroic Spirit. However, Akiha is still capable of ensnaring such foes with her hair with the necessary support.