Len (レン, Ren?) is the main heroine in the main story of Kagetsu Tohya. She also appears in Melty Blood Re-ACT. At the end of Kagetsu Tohya, she formed a contract with Shiki Tohno and is now his familiar.
She once served a Magus several centuries ago, and during that time, the magus helped Arcueid Brunestud defeating a top Dead Apostle by the name of Einnashe, and as a tradeoff for destroying Einnashe's Reality Marble, Arcueid promised to take care of Len after his death. Since then Len's active life has been short, as she only awakened at the same time as Arcueid got out for hunting Michael Roa Valdamjong. But as she was made a long time ago she is growing as a familiar, and also as an animal spirit. In addition, the magus who created Len was extraordinary, so Len has much latent potential. Although she was under Arcueid's charge, they were unable to create a contract, because Arcueid can't make a contract with a spirit of nature, which is the same type as her, therefore Len is not considered Arcueid's familiar. Additionally, Arcueid herself doesn't require a familiar so she won't order it to steal other people's life force and so she just lets it do whatever it wants.
She doesn't speak often, only once briefly in a scene of Kagetsu Tohya and during her interaction with White Len in Melty Blood Re-ACT. She seems to have a liking for cakes, and she dislikes taking baths.
Because she never talks, it's hard to really understand her personality, but basically she's the type of person 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.
In Tsukihime, Len appears indirectly in the "Near-Side route". She gives Shiki a dream of a sexual encounter (which can include any of the heroines: Arcueid, Ciel, Akiha Tohno, Kohaku, or Hisui). She is never mentioned by name. Since she is sent by Arcueid, and Arcueid wishes to know who came to Shiki, an interesting scene occurs if Arcueid is picked. In the Tsukihime Manga Len's silhouette (in cat form) can be seen in the window after he has the dream she sends him.
In Kagetsu Tohya, Len is the main heroine. When the main character, Shiki Tohno, falls into a coma after an accident, she takes it upon herself to keep him alive by putting him through a recurring dream sequence, repeating the same day over and over, his memory wiped clean each day. Things start to get chaotic, however, as Shiki's split psyche, as well as her dangerously, even fatally, low reserves of magical energy, causes things to fall apart. Len puts her all into keeping things in order, while Shiki is slowly starting to gain a realization of his situation. After fighting his other self ("Shiki Nanaya") and confronting his image of death (Kouma Kishima) he meets Len in the courtyard of the Tohno Mansion and is able to form a contract with her.
In Melty Blood and its sequels, Len acts as Shiki's familiar and resides at the Tohno Mansion after the events of Kagetsu Tohya. In Melty Blood Re-ACT, when White Len is first created by Aoko Aozaki as a second TATARI in Misaki Town, Len and Shiki set out to defeat her, though Len enters White Len's realm alone as to settle the score herself. She does defeat her, but does not kill her as she is still 'part of Len' and left her in the care of Arcueid. In Melty Blood Act Cadenza, the two are seen interacting with each other, as White Len intends to form a contract of her own as to gain her own master.
Kara no Kyoukai
Len's black and White Len's white cat form make a cameo appearance as plushes won in a crane machine in the second Kara no Kyoukai movie when Shiki Ryougi's male personality explains the history of himself and the female Shiki.
All Around TYPE-MOON
Len appears in Chapter 2 of All Around Type-Moon alongside several other characters who are attempting to deal with a pot that induces laughter. Len is the only one unaffected and successfully captures the pot, to the rejoice of everyone present.
Len appears in the Tsukihime segments of Carnival Phantasm. Called Black Len (黒レン?), it introduces three other variations of Len in addition to her and White Len. Spotted Len, Calico Len, and Striped Len are introduced in a segment parodying a five-member Super SentaiWP team called Super United Cats Squadron Five Len Cats (超結集猫戦隊ゴレンニャー, Chō Kesshū Neko Sentai Go Ren Nyā?).
Len is a Succubus Demon Familiar with the ability to manipulate dreams, created from infusing the leftover thoughts of a deceased human girl into the body of a dead cat. As such, she is capable of shifting between the forms of a black cat and a prepubescent human girl. As a Succubus she is able to replenish magical energy on her own and she has actually no need to form a new contract, with a new master, to live on.
 Tsukihime Dokuhon PlusPeriod - TSUKIHIME Dictionary: Len [Person's name], p.190-191
Len [Person's name]
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.