Kohaku is one of the two twin maids serving the Tohno family in the anime Lunar Legend Tsukihime. She has lived and worked in the Tohno mansion alongside her younger sister Hisui since childhood, making her a long-time familiar figure to the protagonist Shiki Tohno. Estimated to be around sixteen years old, she wears a traditional kimono as her maid uniform and is distinguished by her pink hair, amber eyes, and a large blue ribbon tied at the back of her head.

Kohaku is consistently depicted with a cheerful and openly expressive demeanor, which stands in direct contrast to her reserved twin sister. This bright and friendly exterior, however, serves as a carefully maintained facade that conceals a deeply traumatic and painful past. Her motivations are intimately tied to this hidden suffering. She developed exceptional skill in creating herbal remedies and poisons, an ability that carries significant weight in the story. Her habit of breaking fragile objects while cleaning is a subtle outlet for the immense inner turmoil she keeps suppressed. Beneath her smile, she harbors a strong desire for genuine happiness and freedom from the shadows of her past.

Within the narrative, Kohaku is a supporting character whose presence is deeply entwined with the dark history of the Tohno household. Her relationship with her twin sister Hisui is a central aspect of her character, a complex bond shaped by loyalty, shared personal tragedy, and contrasting approaches to coping with their harsh upbringing. Her long-standing feelings for Shiki Tohno act as a powerful influence on her actions and are a driving force in her role in the story. She also maintains a complicated dynamic with Akiha Tohno, serving her as the head of the family while quietly harboring a deep-seated resentment and carrying out a secret duty that has major consequences for everyone in the mansion. In the Far Side narrative arc, her true place in the story is fully explored, revealing her as a figure whose suffering is inextricably linked to the family's fate.

The most significant character development for Kohaku occurs within her own narrative arc. For much of her life, she coped with her reality by creating a false, emotionless persona, playing the role of a cheerful doll to survive. In her story, the full extent of her tragic backstory is brought to light, forcing her to directly confront the source of her pain. This confrontation allows her to begin shedding her artificial mask and learning to experience genuine emotion, ultimately guiding her toward seeking a true sense of self and authentic happiness.

Kohaku possesses several notable abilities that contribute to her role. She is a highly gifted apothecary, able to formulate powerful healing medicines and equally potent poisons. Her most unique power is that of a Synchronizer, which allows her to transfer her own vital energy to another person, usually through the exchange of bodily fluids. This ability has profound personal and narrative implications, highlighting both her nurturing qualities and the extent of her personal sacrifice.