He was once the strongest martial artist of Kanto and taught his craft to Traugott Geissendörfer, a famed martial artist who founded the franchise of gyms to which Rakuei Gym belongs.
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. He frequently says that he will kill Izaya after the contract ends.
Sozoro wears a white and black suit with 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 code 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, swords, sand, or combustibles.