Dewgong debuted in A Test in Paradise!, where Goh caught it while he and Ash were on their way to Dragonite Island. Though it was a hard Pokémon to catch, costing Goh all of his Poké Balls, Goh finally caught Dewgong and officially added it to his team. Goh and Ash then rode across the sea on Dewgong, but during their journey, Dewgong spotted another Dewgong, fell in love with it, and chased after it, ignoring Goh's requests to stop. This led the group into a storm, which separated Dewgong from its Trainer.
Later, after being unable to find the other Dewgong, Goh's Dewgong reunited with its Trainer just as he and Ash were about to leave Dragonite Island to follow a group of Dragonite that had just left. Dewgong gave Goh and Ash a lift to the Dragonite, just in time to see one of them being captured by Team Rocket.
In A Snow Day for Searching!, Goh sent Dewgong out to explore a lake where a group of Mankey had dropped a Cubone's bone in. It used its Ice type as a barrier against the cold water while exploring the lake. Eventually, Dewgong found the bone and retrieved it.
In Breaking the Ice!, Goh used Dewgong along with Arctozolt and Ash, Chloe, and Regina's Pokémon to create an ice playground for his classmates. Using Icy Wind along with Regina's Glaceon, Dewgong managed to create a huge structure made of ice that Arctozolt transformed into a castle.
Dewgong was a hard Pokémon to catch in A Test in Paradise!. In fact, Goh had to use up all of his Poké Balls in order to accomplish this task. Dewgong appears to be the leader of the Cerise Park's Water-type Pokémon, with the other Pokémon joining the fray Panic in the Park! only after Dewgong takes the lead. It is quick to fall in love, which often leads to it ignoring its Trainer's commands.
Goh appears to prefer Dewgong for activities that involve swimming. Like other members of its species, it can withstand ice-cold winter waters, and is strong enough to carry its Trainer, Ash, and Pikachu while swimming unhindered.