He is an orange goldfish and one of Stanley's three pets, the other two being Harry and Elsie. In fact, he is Stanley's best friend who gives him education about animals to help him with his everyday problems, which interest him. He also has to point out the flaws in Stanley's ridiculous solutions. He has musical talent and is sometimes shown to be playing the music for songs featured within installments of Stanley. He even breaks the fourth wall in almost every episode by either encouraging the audience to do something fun or by asking them questions corresponding to the given topic.
- Dennis usually spins around before asking questions.
- Dennis made a cameo in an episode of Pinky Dinky Doo. He can be seen in a fish tank in the pet store. Harry, Elsie, Porkchop, Stinky, and Flick Duck appear as well. Just like Dennis, Harry and Elsie, the other three characters are from shows that Jim Jinkins has created (Doug and PB&J Otter).
- It is revealed in the episode "A Boy's Best Friend is His Fish" that Dennis' breed of fish is a Comet.
- In one episode, Dennis says that rabbits are rodents, which is not exactly true.
- He has a similar personality to Cogsworth from Beauty and the Beast.
- He wears glasses to read.
- Dennis is the only one of Stanley's pets mentioned in the show's theme song, which asks, "Who's proud to say his best friend is a fish?"