Lyra's Marill debuted in An Egg Scramble!, where it was found stuck between two buildings by Ash and his friends. When they tried to help it, it used Water Gun to force itself out. Afterwards, Lyra came up to them and revealed that Marill belonged to her. Later in the episode, it teamed up with Khoury's Totodile to defeat Team Rocket and recover Dawn's Pokémon Egg.
In its next appearance in Gone With The Windworks!, Marill overheard Team Rocket saying that they have something to attend to and wanders off after them. Khoury realized that it was missing, and the whole group went looking for it. Marill was later found at the Valley Windworks.
Its final appearance was in A Rivalry To Gible On!. Marill helped Lyra and Ash search for Gible underground using its sensitive hearing. Over the course of the arc, Marill was shown to be very affectionate towards its Trainer.
A † shows that the move was used recently, unless all moves fit this case or there are fewer than five known moves.
In the games
When Lyra is not chosen as the player in HeartGold and SoulSilver, she owns a Marill. It is the first Pokémon she is known to possess. Marill is seen in battle once, when Lyra teaches the player how to catch a Pokémon, and spends the rest of its time following its Trainer. If the player visits Lyra's house, they will see Lyra playing with her Pokémon in her bedroom.
Through phone calls and conversations with the player, Lyra reveals that she thinks her "super-cute" Marill smells like a "dust mop".
In the English translation, she also informs the player that her Marill has started to evolve into an Azumarill, although she is never seen with this Pokémon. In the Japanese version, Lyra only mentions on how "awesome it would be if her Marill evolved".