Kit first appeared as a Lickitung in Licking Lickitung, having stolen Diamond's food and Town Map. Attempting to recover it, Diamond ran into the Pokétch Co. President, who insisted he help advertise his Pokétch. While advertising, Lickitung showed up again in an attempt to grab more stuff, and Diamond decided to capture him. Lickitung attacked Don with Rollout and followed up with a Slam, allowing Don to throw Lickitung. This enraged Lickitung and caused him to evolve into a Lickilicky. Diamond then attempted to capture him, but only ended up upsetting him more. Lickitung then burrowed underground and began attacking with Dig. Using his Pokétch Diamond figured out where Lickilicky's next spot to attack was and had Lax grab him when he surfaced. Diamond then threw a Poké Ball at him, capturing him. Afterward, the two went around town and returned all of Kit's stolen items.
In Yikes, Yanmega!, Diamond ran into a group of Yanmega being controlled by Mars that were attacking Roseanne. After assessing the situation, Diamond had Kit attack with Power Whip to scatter them and find the device that is controlling them. Diamond then had Tru attack with Razor Leaf, but the attack missed, surprising Diamond. Due to this, he and Kit go straight into the middle of the swarm and take one of their attacks head-on, allowing Kit to grab the device and smash it. Due to the device being destroyed, the Yanmega are left without instructions and leave.
In Mixing It Up with Machamp, Kit was separated from Diamond, alongside Tru, at Lake Verity after falling from the Galactic airship. They were later taken to Pearl by Sebastian. In Chancing Upon Chingling, once Diamond was spotted by the Advanced level Grunt, Chatler returns Kit and Tru to Diamond. Kit then participated with Diamond's other Pokémon to battle against the Advanced level Grunt but proved to be no match for his strength.
In Unplugging Rotom (Heat, Wash, Mow, Fan, Frost), Kit battled a wild Rotom that Diamond and Pearl were trying to protect. Diamond sent out Kit to battle against it and had him attacked with a Slam attack alongside Digler's Earthquake, but the moves dealt no damage due to it being an Electric/Ghost type and having the AbilityLevitate. Rotom then moved towards Diamond and Pearl and shocked them while giggling before leaving for Eterna City.
Kit is a mischievous and curious Pokémon who loves to play around. He has a habit of collecting items that he finds, even stealing them if they belong to someone prior to his capture. Kit is a powerful battler and can use his long tongue to deliver strong attacks and swing across trees with great speed. Kit is also easily enraged when losing a fight and will act out aggressively. He has a strong relationship with his Trainer and shares his love of food.
A † shows that the move was used recently, unless all moves fit this case or there are fewer than five known moves.
Trivia
Kit is the only Pokémon that Diamond has ever caught in a battle; Lax, Tru, Don, and Moo were given to him, and Reg and Rotom chose Diamond as their Trainer.