Tokugawa was one of the three great unifiers of Japan, having once served under Toyotomi Hideyoshi. He will not be summoned by Hideyoshi's Transcendent Dazzling Castle of the Sun unlike the rest of Hideyoshi's followers. The real Tokugawa died early on in his reign, but they used the Tokugawa System (徳川というシステム?) of body doubles to hide that fact. The summoned Tokugawa is one of those body doubles.
Much like Hideyoshi is given the moniker of "monkey", Tokugawa is named Tanuki (タヌキ?) due to his appearance.
Tokugawa possesses no Personal Skills or Class Skills on his own, but his Noble Phantasm lets him swap between all seven of the standard Servant classes. Each class has its own status and abilities included with it.