Mr. Tanaka is the elderly steward of the Phantomhive household, upholding traditional butlering standards while overseeing the manor's daily operations. He displays a reserved and dignified demeanor, consistently composed and rarely betraying emotion. His duties involve assisting young master Ciel Phantomhive and coordinating with domestic staff members Mey-Rin, Finnian, and Baldroy.

During the events chronicled in *Book of Murder*, Mr. Tanaka participates in hosting a formal dinner party at Phantomhive Manor, commissioned by Queen Victoria to entertain prominent London underworld figures. When a distinguished guest is murdered and further killings follow, the household staff, including Mr. Tanaka, become suspects due to their access to restricted manor areas and proximity to the crimes. He remains present throughout the subsequent investigation as Sebastian Michaelis and Ciel collaborate with the guests to unmask the murderer.