Felix Yusupov was a Silver Fang Kinfolk. He (allegedly) killed Rasputin on December 29, 1916. However, it is unclear if he did, as several canon sourcebooks describe Rasputin as being a member of several supernatural groups, and as possibly faking his death. See the Rasputin article for more information.
For information on the real person, please see Wikipedia:Felix Yusupov.