Hisui

A sub-heroine in Tsukihime. A servant working at the Tohno mansion. Sympath.
She is a maid who sports an apathetic expression and an inflexible attitude.
Though she seems to be the exact opposite of her older sister Kohaku in that regard, if you look carefully you can see she's actually demonstrating a wide variety of subtle expressions.
Compared to Kohaku, who only ever wears an expression of “happiness,” Hisui actually seems much more human.
She has been working as a servant ever since she was taken in by the Tohno family as a child.
After Shiki left the mansion, she began working there, and stopped talking to people of her own accord.
She's a bit ashamed of it, but it seems that even though she tries her best to maintain conduct befitting a servant when she's in Shiki's presence, every once in a while her true colors slip out.
Just as Kohaku has absolutely no sense for keeping things tidy and in order, Hisui's palate is hellishly out of sync with most people's. The reason she never cooks for Shiki is because she doesn't want to make him sad.
…Geez, that's so cute. I want a maid like that!

Tsukihime Data Collection: Tsukihime Dictionary
§

A servant of the Tohno family, and Kohaku's twin sister. She is in charge of caring for Shiki. Shiki isn't a morning person, so she probably has it pretty tough trying to get him up. She seems quiet and emotionless at first, but if you pay attention she actually displays a greater variety of emotion than Kohaku. She is the complete opposite of Kohaku, so that means she is great at cleaning and tidying, but her sense of taste (food-wise) is terrible, so don't make her spend too much time or effort on kitchen work. Even when she puts her all into making something, the best she could come up with was a plum sandwich (!), but Shiki still ate it seeing as he is that kind of guy. You can find this scene in Kagetsu Tohya.

MB Official Strategy Guide: Melty Blood Glossary
§

A sub-heroine in Tsukihime. A servant working at the Tohno mansion. Sympath.
She is a maid who sports an apathetic expression and an inflexible attitude. Though she seems to be the exact opposite of her older sister Kohaku in that regard, if you look carefully you can see she's actually demonstrating a wide variety of subtle expressions. Compared to Kohaku, who only ever wears an expression of “happiness,” Hisui actually seems much more human.
She has been working as a servant ever since she was taken in by the Tohno family as a child. After Shiki left the mansion, she began working there, and stopped talking to people of her own accord. Also, because of what Kohaku went through as a child, she forbade herself from leaving the mansion.
Though, perhaps it's because her mental fortitude improved during the events of Tsukihime, but in Kagetsu Tohya she actually leaves the mansion of her own accord to bring Shiki his lunch.
She's a bit ashamed of it, but it seems that even though she tries her best to maintain conduct befitting a servant when she's in Shiki's presence, every once in a while her true colors slip out.
Just as Kohaku is incapable of cleaning up inside, Hisui's palate is hellishly out of sync with most people's. It seems the reason she never cooks for Shiki is entirely because she doesn't want to make him sad.
In the Kagetsu Tohya side story Imogirisou, she gets possessed by an evil spirit and demonstrates a strange form of martial arts. This was the inspiration for the Dark Jade Fist, her Arc Drive in Melty Blood.
Her super maid action in Melty Blood is a must see. The way her skirt flutters during her backdash is just… yeah.

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised
§

The younger of the twin sisters working as servants in the Tohno mansion.
The maid assigned to Shiki, she sports an apathetic expression and an inflexible attitude. Though she seems to be the exact opposite of her older sister Kohaku in that regard, if you look carefully you can see she's actually demonstrating a wide variety of subtle expressions. Compared to Kohaku, who only ever wears an expression of “happiness,” Hisui actually seems much more human.
Characterized by extreme fastidiousness and a fear of men. Just as Kohaku is naturally unsuited for cleaning up inside, Hisui's palate is hellishly out of sync with most people's. It seems the reason she never cooks for Shiki is entirely because she doesn't want to make him sad.
Normally she's not a character than can fight, but in Melty Blood due to the Tatari's generous interpretation of a maid as “one who protects their master by any means necessary,” she acquired the legendary Maid-style Killing Arts.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary