Doctor, also known as Doc, is the head surgeon for the Tarantula assassination organization and a central antagonist. He is ultimately revealed to be Lupin III's former partner, who betrayed him years prior by stealing his signature silver Walther P38 and leaving him for dead after a fall from an aircraft.

Within Tarantula, Doctor secretly plots with a faction of discontented assassins to escape the island. He claims to be developing an antidote for a special poison crafted by the organization's leader, Gordo, which requires the island's natural gas to suppress its effects. Doctor states his goal is to steal the organization's stock of gold and flee after deactivating the island's defensive satellite using a program created by Ellen's younger brother, Alex.

These actions conceal a deeper deception. Doctor is actually in a secret partnership with Gordo and possesses the real antidote. After the rebel faction's ship is destroyed, Doctor fakes Gordo's death and helps him steal the actual gold reserve. He then poisons and executes Gordo, believing his partner had become a liability.

His past connection to Lupin drives the plot. The use of the silver Walther P38 to shoot Inspector Zenigata triggers Lupin's investigation. Their final confrontation confirms Doctor stole the weapon and attempted to kill Lupin, and that he was responsible for the death of Ellen's brother, Alex, whom he forced to modify the satellite program and then murdered.

Doctor meets his end during a struggle on an airship, where he is shot by a wounded Ellen with Lupin's assistance. He falls into the ocean, and the silver Walther P38 is thrown into the sea after his defeat.