He was once the strongest martial artist in the Kanto region and taught his craft to Traugott Geissendörfer, a famed martial artist who founded the franchise of gyms to which Rakuei Gym belongs.
Sozoro wears a white and black suit and circular glasses. He is is said to have proper posture and an 'accurate glint' to his eyes, giving him the appearance of a butler or a bodyguard.
He normally speaks humbly and politely, reserving a harsher tone for the likes of Izaya. His humbleness is evident in how he often downplays his own abilities despite evidence to the contrary, such as when he says he has hard time grasping new information in regards to technology – and then has a fluid conversation regarding networking with Nec.
Densuke also possesses a sense of an honor with a respect for humanity and justice. Despite this, he is not above using dirty tactics and will fight with anything on hand, including swords, guns, sand, or combustibles.
Densuke spent ten years in prison for a crime that he did not commit. While in prison, his child's business got into trouble to the point where even his grandchildren were threatened. Izaya proved his innocence and freed him to save his family, simultaneously managing to catch the real culprit of the crime. In exchange for his freedom, Densuke entered into a ten-year contract with Izaya.
Sozoro is first introduced talking with Izaya about the city Bunokura at the Grand Palace hotel. Over the next few days he assumes his role as bodyguard to protect Izaya until he is sent to the Oukarengou's hideout to retrieve Douma Kiyojima under Izaya's instruction. Douma refuses to go along and orders the members of the Oukarengou to attack Sozoro, but he strikes back and swiftly takes down the men and forces Douma to give into their demands. He then escorts Douma the royal suite at the Grand Palace hotel where Izaya was staying.
- Main article: Izaya Orihara
Despite addressing Izaya in a polite and respectful manner, Sozoro is openly hostile toward him. He was aware of the type of person Izaya was but still approached him for help regarding the crimes committed against his family. Because Izaya helped resolve the issue, he feels obligated to fulfill his side of their contract. As such, he frequently mentions he would kill him right away when their contract expires. According to Sozoro, he views Izaya as nothing more than a child with too much power. He has also dislocated Izaya's wrist before.