Len

Len 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 is voiced in Melty Blood ReACT by Kaori Mizuhashi.

Background
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 is she able to replenish Prana on her own and she has actually no need to form a new contract, with a new master, to live on.

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 defeating Einnashe's particular hold, 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 vampires. 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 something that has arrived at the territory of demons, therefore Len is not considered Arcueid's familiar. Additionaly, 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.