Darmanitan first appeared as a Darumaka in Heated Antics!, where he decided to play tricks on the group in order to get their food. He used his incredible rolling skills to avoid the groups attacks and was able to keep them at bay with Flamethrower. Darumaka was eventually challenged by Lewis to a real battle, which he accepted. Darumaka battled against Lewis' Oshawott and proved himself to be a capable battler, using Gyro Ball to reduce damage from Oshawott's Water Gun. However, a well-placed Razor Shell left Darumaka open to another Water Gun, defeating him and allowing Lewis to catch him.
In the following episode, Lewis sent out Darumaka as his first Pokémon in his battle against Hugh. Darumaka faced off against Hugh's Tranquill and lost as he couldn't keep up with Tranquill's speed.
In The Art of Battling!, Darumaka was Lewis' second Pokémon in his battle against Burgh. He battled against Burgh's Shelmet, but was recalled in favour of Sandile after he was unable to breach Shelmet's defences. Darumaka was then sent out to battle Burgh's Leavanny, after it had defeated both Oshawott and Sandile. Despite having the type advantage, Darumaka struggled in the battle after Leavanny restricted his movements with String Shot and hit him multiple times with X-Scissor. Eventually, Darumaka was able to burn through the webs and hit Leavanny with a super-effective Flamethrower, burning it. He then followed up with a Gyro Ball-Fire Punch combination that knocked Leavanny out, earning Lewis his third Gym Badge.
Darumaka was used as Lewis' third and final Pokémon in his battle against Cheren in Lewis Gets Schooled! It battled against Cheren's Watchog and lost relatively easily.
In Insult to Injury!, Darumaka was used in a Double Battle alongside Oshawott against Hugh's Snivy and Blitzle. Despite using a combination of Razor Shell and Gyro Ball, Darumaka was defeated by Snivy's Leaf Tornado.
Darumaka participated in the rematch against Cheren in Re-Class is in Session! He battled against Cheren's Stoutland and managed to land a hit with Gyro Ball. However, Stoutland managed to overpower Darumaka and knock it out.
Lewis sent out Darumaka to battle a stonecutter's Gigalith in Over the Village Bridge! Due to its mobility, Darumaka was able to get in some good hits with Fire Punch and Gyro Ball before being knocked back by Stone Edge. Gigalith then dealt major damage to Darumaka using Heavy Slam, leaving him on the edge of defeat. However, after being encouraged by Lewis not to give up, Darumaka suddenly evolved into Darmanitan. Darmanitan then prevented another Heavy Slam by using Flamethrower, although the damage he had been dealt soon proved too much. To everyone's surprise, Darmanitan activated his Ability and changed into his Zen Mode. Using his newly-learned Psychic attack, Darmanitan was able to lift Gigalith and slam it into the ground, defeating it.
Darmanitan was used as Lewis' first Pokémon in his Gym battle against Drayden in Rise of a Legend! He battled against Drayden's Druddigon and was able to defeat the Cave Pokémon by redirecting its Dragon Pulse with Psychic. Darmanitan was then recalled and later fought Haxorus, but was defeated by Dragon Tail.
In Set Sail for Danger!, Darmanitan was sent out alongside Hugh's Servine to battle against Ghetsis' Bisharp and Hydreigon. During the battle, Darmanitan used its newly-revealed Hammer Arm to send Bisharp backwards, but the battle was interrupted when Ghetsis revealed that he was in possession of Hugh's sister's Purrloin.
Darmanitan later battled against Bisharp once again in A Plasma Civil War!, where it was able to defeat the Sword Blade Pokémon with Flamethrower.
He was Lewis' second Pokémon in his Gym battle against Roxie in Take to the Battle Stage! He battled against Roxie's powerful Scolipede and, after a long battle, they managed to knock each other out.
In Let the Games Begin!, Darmanitan was seen defeating Chris' Druddigon in the preliminaries of the Unova League Victory Heights Conference. He later battled Everett's Accelgor in In the Maractus!, although he was recalled after Accelgor's speed became too much. Darmanitan was later sent out to battle Everett's final Pokémon, Maractus. Maractus' strange movements initially caused Darmanitan trouble, but after transforming into his Zen Mode, Darmnitan forced Maractus to change tactics. This allowed Darmanitan to defeat Maractus with a Psychic-boosted Flamethrower.
In A Full-Strength Rival Battle!, Darmanitan battled against Hugh's Bisharp. During the battle, Darmanitan demonstrated that he had learnt Flare Blitz, using it to easily defeat Bisharp. Darmanitan then attempted to use the move against Hugh's next Pokémon, Chandelure, but it was rendered ineffective by Chandelure's Flash Fire ability. Chandelure then dealt major damage to Darmanitan with Giga Impact, forcing him to transform into his Zen Mode. However, this was part of Hugh's plan, who had Chandelure use a now-super-effective Shadow Ball to defeat Darmanitan.
In Turn up the Heat!, Darmanitan was used to battle Tunja's Beartic. After a clash of Blizzard and Flare Blitz, Darmanitan was able to emerge victorious. In the following episode, he was sent out to battle Tunja's seemingly-unstoppable Metagross, but was defeated without landing a single hit.
Lewis left Darmanitan at Professor Juniper's Laboratory when he decided to go to Sinnoh.
As a Darumaka, he was a very excitable Pokémon, often jumping around uncontrollably and always smiling. It was quite the trickster, playing multiple pranks on the group before being caught. Even so, Darumaka did not show much in the way of an apology, implying that it is oblivious to others' feelings.
Darmanitan's personality changed a bit upon evolution. He became a lot calmer, but had gained a great love of battle, relishing the opportunity to fight a strong opponent.
When sent out of his Poké Ball, Darmanitan always strikes his trademark pose - bouncing on one leg and flexing his arms.