Hau trained his Dartrix, until it fired its quills to bind Ash, Pikachu and Rotom to a tree, suspecting them to be its enemies. After the group cleared the misunderstanding, Ash's Rowlet woke up, and was introduced to Dartrix, who tried to fix its bad hair. The two started a battle; Dartrix used Leaf Blade, but crashed when Rowlet evaded the attack by falling asleep. The two sides used Razor Leaf, but Dartrix's attack was much stronger and negated Rowlet's. Ash went to have Rowlet use the Z-Move, but failed to equip his Grassium-Z. Thus, Rowlet got knocked onto Ash and Pikachu by Leaf Blade, a move that defeated Rowlet. During the rematch the next day, Rowlet used its newly-learned move, Seed Bomb, at Dartrix, hitting it, then went to use Peck. Dartrix retaliated and hit Rowlet with Razor Leaf. After using Leaf Blade, Dartrix bound Rowlet to a tree. Rowlet attempted to attack with Seed Bomb, but his Everstone that he used for the attack was knocked away by Leaf Blade To reverse the situation, Ash empowered Rowlet with Z-Power, freeing itself and using Bloom Doom. The Z-Move defeated Dartrix in an instant.
When Hau met Ash and his friends at the Alola League, he showed them that Dartrix recently evolved into Decidueye. The latter also greeted its old friend Rowlet. Decidueye was then used in a battle against Team Skull, where it used Razor Leaf against Rapp's Zubat. With the first round over, Hau would face Ash in the next round. Hau and Decidueye visited Ash, whose Rowlet started arguing with Decidueye, as it was very inspired to battle it. Decidueye and Hau greeted Ash and Rowlet before the battle. In the stadium, Decidueye fired Razor Leaf, which Rowlet evaded. Rowlet retaliated with Seed Bomb, but got reflected by Leaf Blade. In that moment, Ash and Rowlet attempted to execute the Z-Move Bloom Doom to defeat Decidueye. Luckily, Decidueye was able to avoid the Z-Move and was was able to stop Rowlet in its tracks, by pinning him down with Spirit Shackle. Despite Hau and Decidueye had used their own Z-Move, Sinister Arrow Raid, they only hit Rowlet's cloak, and Rowlet itself was unhurt and ready to continue the battle.
As their battle continued, Decidueye used Leaf Blade and Razor Leaf on Rowlet, who evaded both attacks. He used Seed Bomb, which Decidueye deflected with Leaf Blade. As the two sides clashed, Ash wanted Rowlet to prepare himself to use Brave Bird, but Decidueye blocked him. It used Razor Leaf, which Rowlet dodged, but got bound by Spirit Shackle. Decidueye's used Leaf Blade, to which Rowlet attempted to use Feather Dance, but failed and got knocked out to the wall. Rowlet was able to continue the battle, and as it dodged Razor Leaf, it went upwards to attack Decidueye. However, the latter was faster, and the two sides collided with Sky Attack and Brave Bird. Both Pokémon fell to the ground, though Decidueye managed to stand up. As Rowlet was still lying on the floor, Nanu declared Hau to have won the battle. Hau and Decidueye cheered, but Hala stood up and pointed out that Rowlet just fell asleep. The two sides continued the battle, where the two sides collided using Razor Leaf. Rowlet fired Seed Bomb, which Decidueye reflected with Leaf Blade. As Rowlet went to pick up the Everstone he used for Seed Bomb attack, Decidueye bound him with Spirit Shackle. Rowlet could not escape, so Decidueye used Sky Attack. In that moment, Rowlet used Feather Dance to make a dummy of himself, fooling Decidueye into attacking that. As Decidueye was distracted, Rowlet defeated it with Brave Bird. Hau tended to Decidueye, and both congratulated to Ash and Rowlet for their victory, but Decidueye was a bit concerned over Hau, aware that Hau was on the verge of becoming saddened.
Kiawe and Gladion were thinking of Ash's battle against Hau and Decidueye, and resolved themselves to win their semi-finals battle to face Ash in the finals. When the MC described Ash's victories in the Alola Pokémon League, he reminisced about Hau and Decidueye. When a Guzzlord descended from an Ultra Wormhole at Manalo Stadium, Hau sent Decidueye to assist Gladion's Lycanroc and Kiawe's Turtonator at battling it. Decidueye fired Razor Leaf to target Guzzlord's second face - its weak spot - but Guzzlord blocked the attack with its claw. Instead, Decidueye used Spirit Shackle to bind Guzzlord's claw, but it still deflected the rest of the attacks, and uttered a roar to blow them away. Suddenly, Lillie, Sophocles, Lana and Mallow arrived to help the boys in fighting Guzzlord. Hau had Decidueye use the Z-Move, Sinister Arrow Raid, which merged with the rest of the Pokémon's Z-Moves, creating a strong enough blast to push Guzzlord back through the Ultra Wormhole. The next day, Hau and Decidueye watched the battle between Ash and Professor Kukui, and were happy to see Ash having won that battle.
With Ash's departure from Alola region, Hau, supported by his grandfather, took on Nanu's Grand Trial, using his Decidueye against Nanu's Persian.
- Shin-ichiro Miki (Japanese)