Denis

Dennis is an old friend of Simon Brezhnev and the owner of Russia Sushi. He works inside Russia Sushi as the chef, while Simon usually stands outside attracting customers.

Characteristics
Dennis is a large white man usually seen working behind the counter as a chef for Russia Sushi. As such, he is always seen wearing the Russia Sushi uniform. As a result of his past in the Soviet Union, like Simon, he is a pacifist, and tries to stop conflict before they start.

Although his strength is unknown, he is shown to have significant skill with knives, with both skills acquired as a sushi chef and possibly from battle. When Masaomi Kida and Kyohei Kadota were discussing the Yellow Scarves vs. Dollars war, as tensions rose Dennis threw his knife into the scroll on the wall, narrowly missing his customers (but slicing off a bit of Togusa's hair) and freaking them out. As it is unlikely he actually tried to kill any of them, this indicates that he has very good aim.

Anime
Anime specific spoilers about this character per episode.

Episode 01:

Episode 02:

Manga
Manga specific spoilers about this character per volume/chapter.

Volume 01:

Light Novels
Light Novel specific spoilers about this character per volume/chapter.

Volume 01:

Volume 04 : Mairu and Kururi stumble into the resturaunt with an ijured Igor and they immediately bring him to the back. Dennis, Simon, and Igor appear to be aquaintances and agree to help him. Dennis calls Shinra over to treat his wounds but he isn't able 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, Dennis, Simon, and Igor agree to help the Orihara twins meet Celty. He stuffs Igor into a bag and has Celty transport him for an entire day across the city. Dennis expects Celty to eventually reach her home where Igor will inform the twins of it's location. After the Toromaru members are taken care of, Igor tells Dennis and Simon of his desire to take a vacation from work.

Volume 05 & 06 : Dennis and Simon appear in Vorona's flashback. They were apparently old friends of her father, Drakon, and agreed to teach 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. At some point afterwards, they immigrated to Ikebukuro.

After Vorona and Slon retreat to their hideout and then defeated by the awakusu, Dennis, Simon, and Igor appear and reprimand the two of them for causing such chaos. Simon takes an unconcious Vorona back to the resturaunt while Slon is taken by the Awakusu.