Who is Prince Felix Yussoupov from Nicholas and Alexandra and what is their importance?

Prince Felix Yussoupov was the husband of a niece of the Tsar. He was also the wealthiest man in all of Russia. Having been to Rasputin for his healing services, Felix concludes that Rasputin is a big danger to the monarchy and must be killed. He enlists several co-conspirators who decide to poison Rasputin.