Hisui's twin sister who along with her sister is one of the maids in the Tohno Mansion and one of the five heroines in Tsukihime
Kohaku is a member of the Fujou family who was brought along with her sister Hisui to live with the Tohno family. This is due to the Fujou family having the power to transfer energy that the Tohno family's head, Makihisa, needed to maintain control of his powers, as the ability was strongest during sexual intercourse. As a result, Kohaku was regularly raped by Makihisa as a child. She was one of Shiki's playmates as a child whom gave him her hair ribbon before he left to live with the Arima family, making him promise to return it.
Later on, Makihisa decided to give Kohaku some more freedom under the condition that she took care of his imprisoned son, SHIKI (who was still alive following the incident with Shiki and whom Makihisa didn't want to kill). But due to having to maintain control of his body as well, SHIKI started to rape Kohaku as well. This drove Kohaku into despair and led her to develop the desire to seek revenge on the Tohno family. To do so, she fed medicinal herbs to brainwash and manipulate SHIKI and led him to kill Makihisa. These actions would indirectly lead to Shiki to reunite with Akiha at her mansion.
Realizing the problems that plagued Kohaku, Hisui and Akiha grew depressed over the matter as they allowed Kohaku to become a maid at Akiha's mansion. With Hisui's depression, Kohaku decided to take on Hisui's role until she became cheerful, unaware that this only burdened Hisui more.
Kohaku stands at 156 cm (roughly 61 inches) tall with her three sizes being 78 cm for bust, 58 cm for waist and 80 cm for hips. She has short red hair and yellow eyes. Kohaku is normally seen wearing a blue hair ribbon and a brown traditional Japanese kimono while serving as a maid at Akiha's mansion.
On the surface, Kohaku is a cheerful and clumsy girl with the habit of breaking many objects around Akiha's house. However, she still secretly holds feelings of hostility for the Tohno clan due to her tragic past. While claiming to hate Shiki for their silent connection for one another as children, she is secretly in love with him and is part of the reason why she chose to take on Hisui's role. She is very knowledgeable in the use of medicine, which she learned of from one of Makihisa's acquaintances named Jinan Sougen.
Kohaku's role is minor in the Tsukihime visual novel for the routes of Arcueid and Akiha. She does have an expanded role in the anime where she provides Shiki with information that allows him to learn more about his past and helps Akiha control her demon blood by allowing her to suck on blood from her breast.
In Ciel's route in the visual novel, Kohaku winds up being brutally beaten and raped by Shiki when he is possessed by Roa's soul, with the actions leading a horrified Shiki to flee from Akiha's mansion.
In Hisui's route, Kohaku has a more active role where she is manipulating everyone in Akiha's mansion to fulfill her plans of revenge on the Tohno family by killing Akiha. She drugs Shiki throughout the route, leading him to question his sanity. Despite her actions leading Shiki to rape her in his drugged state, this doesn't stop her from seeking revenge. When Akiha is later killed by SHIKI, the event shocks Kohaku as she comes to learn that Akiha wanted to atone for her family's sins upon learning of Kohaku's past and Kohaku realizing she came to care for Akiha as a close friend. The shock drives Kohaku to want to kill herself. Depending on the ending for the route, either Kohaku goes through with her suicide by stabbing herself in the chest with a knife soaked in poison or is saved by Shiki and left with amnesia which makes her forget her past.
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. She used to have a more cheerful and outgoing personality, yet took on a more subdued personality for Kohaku's sake from her tragic past with Makihisa.
At a welcome party celebrating his return to the Tohno family, Shiki goes outside to cool down from drinking when he meets Kohaku. He tells her about the quiet girl from his past and Kohaku warns him that he will face suffering for so long as he stays at the Tohno mansion.
Shiki later learns that Kohaku was the girl who gave him the ribbon and that Akiha has given into bloodlust due to gaining SHIKI's burden of madness from the slaying of her brother. Kohaku believes that Shiki reciprocating Akiha's feelings for him is the only way to calm her. Shiki refuses to do this because of his love for Kohaku and returns the ribbon to her. This leads Kohaku to cry and give up on her plans of revenge on the Tohno family. The two have sex and Kohaku offers to Synchronize with Shiki to give him the power needed to stop Akiha. Instead, she attempts to put Shiki to sleep with a sedative so she can confront Akiha alone. However, Shiki is able to resist the drug's effects.
At the school, Shiki confronts Akiha, who acknowledges the suffering that her family put Kohaku through and still assumes that the maid is out for revenge on the Tohno family. Shiki reveals that he still loves Kohaku, driving Akiha to attack the latter with her hair. These events lead Shiki to try attacking her, yet he can't bring himself to kill Akiha. Kohaku appears unharmed and reveals she no longer bares hatred for the Tohno family, with Akiha revealing that she held back in inflicting any harm on the maid.
In Kohaku's only ending, she forgives the Tohno family and goes on vacations with Shiki in the Nanaya forest.
Kohaku lacks any abilities for combat. She was born as a Synchronizer, a person with the ability to transfer energy to someone through bodily fluids. While the transfer works for both genders, it is most effective in sexual intercourse with a man.