Kohaku's twin sister who along with her sister is one of the maids in the Tohno Mansion.
Hisui used to be a member of the Fujou family before she and her sister Kohaku were taken in by the Tohno family as children. The Tohno clan's leader, Makihisa, needed an ability from the sisters that allowed them to supply energy to others in order to maintain control of his body.
Hisui never found herself forced to use her abilities due to Kohaku pleading with Makihisa not to be abused. As a result, Hisui lived a fairly normal and pleasant life and was one of Shiki's playmates when he was first living with the Tohnos.
Upon learning of her sister's abuse under Makihisa though, Hisui grew to be quite depressed and Kohaku chose to take on her sister's identity until she was cheerful again. However, this only burdened Hisui more.
Hisui stands at 156 cm (roughly 61 inches) tall with her three sizes being 76 cm for bust, 58 cm for waist and 82 cm for hips. She has short red hair and blue eyes, normally wearing a bonnet on her head and a brown western maid dress.
While appearing as a stern woman on the surface, Hisui is actually a kind and loving girl forcing herself to restrain her emotions to keep Kohaku's emotions as they are due to her past with Makihisa.
Hisui's role in most routes of the Tsukihime visual novel are minor as she only serves as Akiha's maid at her mansion. She does get a limited role in Kohaku's route when she reveals to Shiki that Kohaku was the one who gave him her hair ribbon when they were children.
Returning to the Tohno mansion after eight years, Shiki recalls the times where he played with Hisui and Kohaku as children, with one of them being a quiet girl who gave him a ribbon. However, he mixes the two up as he believed Hisui to be the mentioned quiet girl.
At Shiki's welcome party, Hisui would pass out after drinking a glass of wine due to being unaccustomed to drinking. Shiki would carry her to his room to rest and would also fall asleep, dreaming about the girl who gave him the ribbon. The following morning, Shiki sees Hisui sleeping on his chest. As the route progresses, Hisui grows more comfortable around Shiki and reveals more of her true personality to him, yet doesn't reveal Kohaku's plans to destroy the Tohno family out of revenge for her tragic past with Makihisa.
After SHIKI Tohno drains Shiki of his life energy, Hisui offers to Synchronize with the latter to defeat SHIKI. After this, she would start to accompany Shiki to school.
In Hisui's "True End", Kohaku reveals that she killed Akiha and SHIKI and commits suicide. Before dying, she reflects that she has no regrets for her actions as they helped to save Hisui from Makihisa. Hisui would begin a relationship with Shiki and accompany him on his vacations, throwing her sister's ribbon into the sky before leaving with him.
In Hisui's "Good End", Kohaku reveals her involvement in the Tohno family's downfall and claims to hate Shiki for their silent connection together. Hisui appears and reveals that her sister is lying as she loved Shiki and Akiha. If Kohaku truly wanted to take on Hisui's personality, then loving Shiki is an integral part of Hisui's personality. Hearing these words, Kohaku attempts to kill herself with poison yet is saved in time by Shiki. She loses her memory afterward and is renamed "Nanaya" by Hisui, as she returns to being a maid at the Tohno mansion. Later on, Shiki and Hisui would go to Hisui's favorite spot in the city together.
Regardless of the ending, Hisui would regain her cheerful personality.
Hisui lacks any combative abilities. She is a Synchronizer, a person with the ability to transfer energy to another person through bodily fluids. While this ability works with both genders, it is most strongest during sexual intercourse with a man.