Len

Arcueid's familiar. A succubus. Normally found hanging around Arcueid in the form of a black cat.
She is a succubus that can enter other people's dreams, and is responsible for causing the wet dream Shiki had about Arcueid, Ciel, Akiha, or Hisui.
Though she does not appear in the original work, she is a young girl of about ten years old.
She has calm expressions, and wears a black coat with a big ribbon in her hair. It's no exaggeration to say that her charm is enough to completely overshadow the other heroines.
She even had character illustrations and sex scenes finished and ready to go, but in the end they didn't make it in.
She was going to be a secret heroine in Tsukihime, but we had to throw that idea out due to scheduling issues.
If you happen to have a copy of the bonus book that came packaged with the Half-moon edition, you can actually see her listed on the sub-characters page.

Tsukihime Data Collection: Tsukihime Dictionary
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Arcueid's familiar. A succubus. Normally found hanging around Arcueid in the form of a black cat.
She is a succubus that can enter other people's dreams, and is responsible for causing the wet dream Shiki had about Arcueid, Ciel, Akiha, or Hisui.
She does not appear in the original work, but her outward appearance is that of a young girl of about ten years old. She has calm expressions, and wears a black coat with a big ribbon in her hair. It's no exaggeration to say that her charm is enough to completely overshadow the other heroines.
She appears as a guest heroine in Kagetsu Tohya.
…Well, she wasn't so much a guest as she was the core of the story, actually.
Though at first it may seem that she is unable to speak, her voice box is intact and she can understand human language. Apparently she has simply decided not to talk for some reason. In Re-ACT, she only has a single spoken line.
Also, her pixel art in Re-ACT is, in a word, amazing. If she had a voice to match, she'd probably be able to conquer the entire world.
Because she never talks, it's hard to really understand her personality, but basically she's the type that likes to quietly spend her time alone. Just like a cat, she is haughty and fickle, but prone to sudden bursts of attention-seeking craziness as well.

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised
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Arcueid's familiar. A succubus. Normally found hanging around Arcueid in the form of a black cat.
A guest heroine in Kagetsu Tohya, she was the one responsible for creating the eternally looping day. She is a succubus that can enter other people's dreams, and due to various events is currently living at both the Tohno mansion and Arcueid's apartment.
In short, Arcueid is her contractual master, while Shiki is the master she chose herself.
Though at first it may seem that she is unable to speak, her voice box is intact and she can understand human language. Apparently she has simply decided not to talk for some reason.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary
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