Marill along with Tracey, as seen in the official artwork
Marill was first called upon in The Crystal Onix to help locate the mythical Crystal Onix using its superior hearing abilities after Venonat failed to locate it with its sight. In a matter of seconds, Marill picked up on Onix's voice and led the group to its secret den located on the other side of Sunburst Island. As a Water Pokémon, it was also able to find Onix hiding underwater.
On the day that Tracey announced that he would leave the group in order to be Professor Oak's assistant, he used Marill to help send Team Rocket blasting off after Scyther had rescued Pikachu from them.
Marill ready to battle alongside its child, Azurill
Generally, Marill is shown to be a friendly Pokémon with nurturing instincts. While the group was on a deserted island in A Way Off Day Off, it used its tail to keep Togepi entertained. Marill is also generally obedient unless it is faced with an opponent that is big and scary, as seen in In the Pink. In such instances, Marill will jump in Tracey's arms for protection.
Marill helps Tracey as a Pokémon watcher as its ears can hear Pokémon from far away and locate where the Pokémon are found. He has also used it to help find Ash's Pikachu and Misty's Togepi when they became separated from their Trainers.