She is the older of the two twin maids in the Tohno mansion. Her name, Kohaku, is the Japanese word for amber, referring to her eye color.
Kohaku is a maid of the Tohno Mansion and twin sister of Hisui. When a rumor that one of the Tohno family servants was planning to take over the mansion manifested on The Night of Wallachia; Kohaku was selected as a target for the Tatari curse. As a result, she gained her clever mastermind, devious schemes, and scientific knowledge which allowed for the creation of Mech-Hisui. Thusforth, Kohaku spends most of her off time scouting for new people to aid her in her takeover of the Tohno Mansion, and looking for any helpful traps or devices along the way.
Kohaku has short red hair, like her sister Hisui. She wears a blue ribbon at the back of her head and wears a brown Japanese kimono.
Kohaku is always seen to be smiling and cheerful, being especially gifted with medicine which she learned from Sougen Jinan. She has a habit of breaking many (and valuable) objects when cleaning indoors. Though she always states that it was not intended, some characters, like Akiha, seems to think otherwise.
However, all of it is an act. In truth, Kohaku’s smile is empty, as she was mentally scarred and traumatized at a young age due to constantly being raped by Makihisa. Eventually, she stopped feeling emotions, and put up a false smile as well as a cheerful demeanor. In most of the routes of Tsukihime, she continues this facade after the routes end. However, in her own route, she manages to forgive the Tohno family, and eventually truly becomes a cheerful girl.
She claims to have hated Shiki for the silent connection they shared as children (watching one another but never meeting), but it is revealed that she is actually in love with him, and it is in part why she wanted to take on Hisui’s role in the first place. Kohaku simply wanted to innocently love the boy named Tohno Shiki, just as Hisui did as a young girl.