White Len's contract with Nanaya (this is the supressed personality of Tohno Shiki, from Tsukihime) is a symbiotic (yet seemingly contradictory) one -- she maintains his existence with her TATARI powers, while he supplies her with the prana she needs to survive.
However, in the White Len ending in Act Cadenza, Nanaya clears states that he is in the "master" ...
to answer the question in the comments:
melty blood react final tuned -> actress again is the full storyline (of melty blood, melty blood itself being a sequel to tsukihime)
melty blood original and react/react final tuned have the exact same story.
i believe more people may be looking for this question due to recent developments so heres the answer.