Denis is a large white man usually seen working behind the counter as a chef in Russia Sushi. As such, he is always seen wearing the Russia Sushi uniform.
As a result of his past in the Soviet Union, Denis, like Simon, is a pacifist and tries to stop conflicts before they start. He appears wise and well-connected, as he is able to offer sound advice and has the contact information of Izaya and Haruya Shiki. Unlike Simon, Denis speaks fluent Japanese, but he speaks up less than his partner.
Although Denis' strength is unknown, he is shown to have significant skill with knives, skills acquired both in battle and as a sushi chef.
Not much is known about Denis' past other than he and Simon were former special ops soldiers for the Soviet Union. Under Drakon, they trained his daughter Vorona in combat and weapons. Growing weary of a life of fighting, along with the collapse of the Soviet Union, he and Simon immigrated to Japan and opened a restaurant and have remained there for several years.
Mika Harima Arc
Denis is shown working inside Russia Sushi whenever other characters come in to eat.
Shuuji Niekawa visits Russia Sushi asking Simon who the toughest guy in Ikebukuro is. Denis tells Niekawa that Simon does not talk about fighting and sends him to talk to Haruya Shiki.
Yellow Scarves Arc
Denis throws a knife into the room where the van gang and Masaomi Kida are talking about the possibility of gang war. The knife cuts Saburo's hair before sticking into the wall, and Denis tells them to not talk about gangs in the restaurant. Outside, Walker and Erika theorize that Denis is a former Soviet agent.
Mairu and Kururi stumble into the restaurant with an injured Egor, and they immediately bring him to the back. Denis, Simon, and Egor appear to be acquaintances and agree to help him while Egor tells them his reason for being in Tokyo. Denis calls Shinra over to treat his wounds, but he is unable to afford Shinra's fee of 200,000 yen. Mairu and Kururi offer to pay without a second thought.
In return for helping them with the bill, Denis, Simon, and Egor agree to help the Orihara twins meet Celty. He disguises himself and stuffs Egor into a bag, asking Celty to transport him for an entire day across the city. Denis expects Celty to eventually reach her home or another stopping point, where Egor will inform the twins of his location. Things do not go according to plan, and after the Toramaru gang members pursuing Celty are taken care of, Egor tells Denis and Simon of his desire to take a vacation from work.
Denis and Simon appear in Vorona's flashback. They were old friends of her father, Drakon's, and taught her self defense. Vorona excelled at the training and eventually expressed a desire to learn how to use weapons. They initially refused but eventually gave in to her demands, and at some point afterward, they immigrated to Japan.
Dollars and Blue Squares Arc
After Vorona and Slon retreat to their hideout and are ambushed and defeated by the Awakusu-kai, Denis, Simon, and Egor arrive and reprimand the two of them for causing such chaos. Simon takes an unconscious Vorona back to the restaurant while Slon is taken by the Awakusu.
Daily Life Arc
Denis gives Vorona a job at Russia Sushi.
Final Curtains Arc
He, Tom, Simon, and Kine end up being trapped in Russia Sushi by the Saika Army.
Denis overhears Yahiro and Kuon talk about Shizuo and warns them to stay away from him.