Prior to the start of the Diamond & Pearl chapter, 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 Passing by Probopass and Maneuvering around Magnezone, the group enter Mt. Coronet and encounter the leader of Team Galactic, Cyrus. He has his Probopass attack them with a powerful Rock Slide for being "noisy" around the site of his Nosepass and Magneton's evolutions, and Pearl and Platinum become trapped under the rubble. The Luxio they met before shows up and uses his X-Ray vision to find the two that were buried, and has Lax and Tru use Tackle to help break the two out.
In Stunning Staravia & Stinky Skuntank II, Lax fought against some Team Galactic Grunts in order to stop them from capturing Platinum. He is seen attacking a few Staravia.
In Dramatic Drapion & Crafty Kricketune II, Lax battled two Scientists that were attacking Diamond and Platinum. At first Lax hid himself and avoided the fight, but eventually joined and attacked with Fling while riding on Ponyta. Once they became trapped he uses Rollout to defeat them.
In Problematic Probopass & Mad Magnezone II, Lax battled Cyrus. Diamond had Lax steal the camera Cyrus was using by activating Pickup. At first, Lax was able to trick Cyrus's Magnezone by taunting it with the many Berries hidden in his fur, but is defeat when Magnezone uses its Magnet Pull Ability to pull all metallic objects to it.
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.
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.
In Double Trouble with Dialga and Palkia VI, Lax was used alongside the other Pokémon to battle against the Legendary Pokémon Dialga and Palkia. Despite his small size, Lax was able to do great damage to the Dragons with the help of Diamond and Pearl's Pokémon and with a combined technique, they were all able to destroy the Red Chains controlling them.
In The Final Dimensional Duel XI, Lax tried to stop Charon from closing the portal to the Distortion World with his machine. Charon quickly used the machine to control Lax and had him attack his own Trainer instead. Rotom then showed up in a Proteam Omega toy and destroyed the machine.