Prior to the start of the series, Lax was given to Diamond as an Egg and eventually hatched. He first appeared in Stagestruck Starly, performing in Diamond and Pearl's comedy routine in Jubilife City. Lax managed to ruin the skit by eating rice balls instead of doing his part of the act, much to Pearl's frustration. Despite ruining the act, the audience found it funny, allowing Diamond and Pearl to win the special prize. Later the two are attacked by a group of Starly and Lax helps fend them off alongside the other Pokémon.
In Mirages of Mismagius I, Lax is used in Platinum's Gym battle against Fantina. He starts off by fighting her Duskull and defeats it with a single Lick attack and is returned. He is later sent out to fight her Mismagius after Ponyta and Empoleon were defeated. Due to Mismagius's illusions Platinum became unable to give a command and Lax became distracted. With help from Pearl the two get in sync and Lax attacked with Fling, which was enhanced because of an Iron Ball, defeating Mismagius.
In Lucky Lucario I, Lax is trained on Iron Island by Riley alongside Tru and Don. The three start by battling against Riley's Lucario and are quickly defeated due to Lucario's speed. Despite the loss Riley points out that the three have amazing defense and work wonderful together. After this Riley took the group into a cave and they are attacked by a wild Steelix. Lax and Don manage to hold it down long enough for Tru to attack with Crunch, making the Steelix flee. After the training Lax learned to overcome his weakness in speed.
Lax and Chatler
In Chancing Upon Chingling, after Diamond arrived on the Galactic Veilstone Building due to failing to stop Mesprit from being captured by Team Galactic, Lax is used to help sneak around the base. Diamond uses Lax's Berry picking skills to trip and knock out a Grunt so he could steal his uniform to move about the building undetected. Once Diamond was spotted by the Advanced level Grunt, Lax participated with Diamond's other Pokémon to battle him but proved to be no match for his strength.
Like his Trainer, Lax is gluttonous, lazy, relaxed, and always seen eating. He is a slight coward and freaks out at the sight of conflict and sometimes attempts to hide from it. Despite being a baby Pokémon with little training, he shows considerable stamina and durability. Lax has the ability to store many items inside his fur, such as Berries or an Iron Ball.