Kohaku

A sub-heroine in Tsukihime. A servant working at the Tohno mansion. Sympath.
An energetic girl, that always sports a bright, unwavering smile. In the story, she was the servant assigned to Akiha, and was in charge of cooking and tending to the gardens.
She has a natural cheerfulness about her, and just spending time with her is relaxing. On the other hand, she is also very cool-headed and intelligent—though she isn't even aware of it herself.
She was taken in by Tohno Makihisa together with Hisui, and spent her days confined in Makihisa's room.
Around the time that Akiha entered her second year of middle school—after Shiki was put up for adoption—she found out about Kohaku's relationship with Makihisa and made her a normal servant working on the mansion grounds.
As a character, she represents “the lower ruling the higher.”
She was supposed to be a sub-character to the bitter end, but could end up being the most popular of all if we're not careful.
Now, this is supposed to be a secret, but originally Kohaku didn't even have a route.
What changed that, you ask? Well, after we released the Half-moon edition we read through the player feedback, and the one thing that almost everybody mentioned was that Kohaku needed a route too.
……Of course, there was some resistance. As far as the scenario director was concerned, Hisui and Kohaku were a paired set, so something like individual routes was totally impossible, and on top of that the scenario programming and event graphics for the Hisui and Akiha routes still weren't even finished even though the winter Comiket was only three months away! It was only due to the art director's passionate arguments that we decided to attempt the impossible in the end.

Tsukihime Data Collection: Tsukihime Dictionary
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The craziest strategist serving the Tohno family. From time-to-time you can see her transform into Magical Amber, or maybe not. She always seems to be smiling and laughing… She has great cooking skills, but avoids cleaning like the plague. She is the complete opposite of her younger sister Hisui. She is also the one mainly responsible for taking care of Akiha.

MB Official Strategy Guide: Melty Blood Glossary
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A sub-heroine in Tsukihime. A servant working at the Tohno mansion. Sympath.
An energetic girl, that always sports a bright, unwavering smile. In the story, she was the servant assigned to Akiha, and was in charge of cooking, tending to the gardens, and managing the finances of the estate.
She has a natural cheerfulness about her, and just spending time with her is relaxing. On the other hand, she also has an extremely cool-headed and objective point of view—though she isn't even aware of it herself.
She was an orphan taken in by Tohno Makihisa together with Hisui, and spent her days confined in Makihisa's room. Then, around the time that Akiha entered her second year of middle school—after Shiki was put up for adoption—she found out about Kohaku's relationship with Makihisa and made her a normal servant working on the mansion grounds.
As a character, she represents “the lower ruling the higher.”
Supposed to have been a sub-character to the bitter end, but could end up being the most popular of all if we're not careful… is what the staff thought. Wait, that's not right, hasn't there always been blatant favoritism?
In reality, when development first started there wasn't a route planned for Kohaku at all. What changed that, you ask? Well, after we released the Half-moon edition we read through the player feedback, and the one thing that almost everybody mentioned was that Kohaku needed a route too. So we caved to pressure and tacked it on at the very end.
Although she was a tragic heroine in Tsukihime itself, ever since then she's just been running around letting off steam.
In Kagetsu Tohya, she gathered popularity as a mischievous housekeeper, and showed the first signs of her budding talent as a mad chemist. Her exploits included building a secret room in the Tohno mansion's basement, wearing a hood and calling herself a magical girl, and even taking part in a bad end involving anesthetic drugs.
In Melty Blood, since she's more-or-less the Kohaku from Kagetsu Tohya and not the one from Tsukihime, her magical-ness received an upgrade. Though the unbalanced Kohaku is nice, the carefree Kohaku from Melty Blood that does as she pleases is pretty nice too, don't you think?
……This is a digression, but apparently there are a lot of users that have been completely defeated by Kohaku's voice in Melty Blood. In fact, even the scenario writer was eviscerated.

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised
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The older of the twin sisters working as servants in the Tohno mansion.
An energetic girl, that always sports a bright, unwavering smile. In Tsukihime, she was the servant assigned to Akiha, and was in charge of cooking, tending to the gardens, and managing the finances of the estate.
In contrast to her natural cheerfulness, she also has an extremely cool-headed and objective point of view that even she herself is not aware of.
She was an orphan brought in to the Tohno mansion together with Hisui. She has the bad habit of teasing her master, Akiha, even as she looks after her.
Furthermore, she is a skilled pharmacist, and often lends her aid to the anemia-prone Shiki. Though she is indeed well-versed in strange medicines due to her job, that fact was exaggerated by the Tatari, leading to her appearance as the extremely mysterious Magical Doctor Kohaku in Melty Blood.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary