The new characters that were introduced are Beamer, Reflex, Tammy, Jeff, Marvin McNasty, Lumpy, Bones, Collette, Big Paw, Charlamange, Hairball, and Whopper's niece and nephew. Katrina Stoneheart, Brattina, Holly, and Catgut did not appear in this movie. The Pound Puppies, returning from the TV series, were redesigned for this movie, which is the reason why none of the Pound Puppies look like their original selves in the TV Series.
Holly was originally going to be a character in this movie as the pound owner. That idea was scrapped by the production crew because they didn't think an 11 year old girl running a puppy pound would be believable to movie-goers. So Tammy and Jeff were created. They thought young adults running the pound would be more believable.
In an early draft, Katrina Stoneheart was going to reprise her role as the villain. She was later written out of the movie because the story writers wanted a villain who wanted to gain "puppy power" and use it to control the world. All of the writers thought it wouldn't work if Katrina was used, so Marvin McNasty was created to replace her as the villain. Since Katrina was written out, Brattina and Catgut also had to be written out.
Ruth Buzzi, Nancy Cartwright, and BJ Ward were the only actresses to reprise their roles as Nose Marie, Bright Eyes, and Whopper from the TV series respectively. Cooler (originally voiced by Dan Gilvezan) was voiced by Brennan Howard (with Ashley Hall providing his singing voice) and Howler (originally voiced by Robert Morse in the TV series) was voiced by Hal Rayle (who also did the voice of Reflex in the same movie).
Cooler now wears a white leather jacket and no longer sports his trademark Eddie Murphy-style laugh. During the "Let's Go to the Pound" segment, he wears his favorite pair of sunglasses. He is somewhat more serious than he was in the TV Series.
Whopper is older than he was in the TV Series. Instead of wearing a diaper, Whopper wears a blue baseball cap, a blue shirt, and blue pants. In the present, he is now grown-up, wearing a black jacket and has a niece and nephew.