Character: Maril (Miyuki)

Marill is an aquatic mouse Pokémon. Marill's tail floats like a buoy because it is filled with oil, which is less dense than water. When fishing at a fast-running stream, Marill wraps its tail around a tree. Since the tail is stretchy and flexible, it allows the Pokémon to get food and still stay safely anchored. Otherwise, when in a lake, it uses the buoy-qualities of its tail to keep itself suspended as the rest of its body dives underwater to feed on aquatic plants. Its fur is impermeable, which keeps it dry, and that allows it to swim through water at high speed without being slowed by the water's resistance.

In the Pokémon Anime, Tracey, the Pokémon Observer from the Orange Islands, has a Marill which he uses on multiple occasions.

Max borrowed Ash's Corphish to fight against Vivi and her Marill.

Pietra used her Marill to battle and defeat Rhydon.

Trixie's Marill performers in Trixie's Pokémon circus.

Tracey's Marill was revealed when Tracey decided that he needed a good Pokémon to help him find the Crystal Onix. Tracey's Marill has very sensitive ears that can detect distant sounds and the round, rubbery tip of its tail can expand and contract, assisting Marill in the water. While swimming, Tracey's Marill tail floats so he can see where it's going.

Misty realized that Tracey and his Marill make a good team as Tracey's Marill could hear a Pokémon long before Tracey could even see it. Tracey would often play a Pokémon cry on his dex for Marill to help it learn what the Pokémon sounded like so it could search for that particular Pokémon. Tracey's Marill ears would twitch when it was searching or it found something.

Lyra's Marill was stuck between two buildings and Ash's Pikachu and Dawn's Piplup rescued it just as Lyra arrived and introduced herself.

Maril is Lyra's main Pokémon and it helped rescue Dawn's egg from Team Rocket with its powerful Water Gun. It also uses its Water Gun as a location tracker by shooting it high in the air for others to see.

Lulu's Marill helps Lulu run a berry farm near Suiren Town in the Sinnoh region. Dawn's Pochama found Lulu's Marill attractive and tried to impress it until a wild Elekid intervened. Marill was captured by Team Rocket as they were spotted by Marill attempting to steal berries from the farm. Pochama and the wild Elekid managed to help rescue Marill from Team Rocket.

As Dawn was leaving, her Pochama got advice from Brock telling it to be strong as it could see that Elekid and Marill were becoming closer friends.

Lola's Marill battled at the Pewter Gym.