Decidueye first appeared as a Rowlet in The Delivery of Rotom and the Girl. Professor Kukui was seen testing Rowlet and Popplio's moves in his lab, causing a ruckus that sent all three flying through the roof and onto the beach, where they met Moon and Sun. While Sun introduced Moon to Kukui, Popplio sneezed, trapping Rowlet and Kukui in its water balloons. Though Moon immediately freed Kukui with one of her arrows, Popplio had to remind her to also rescue Rowlet. Due to spending so much time in the bubble, Rowlet developed a cold and had to be healed with Moon's medicine. Afterward, Rowlet took a liking to the girl, the two forming a bond because of their mutual preference of nighttime.
In Defeat and the Commander of the School of Fish, Rowlet was used to battle a Xurkitree that attacked Tapu Lele at Lush Jungle. Rowlet kept Xurkitree distracted long enough for Moon to shoot a Poké Ball-tipped arrow at it. Though Xurkitree escaped being captured, Tapu Lele was freed from its grasp, after which it angrily flew off. Afterward, Rowlet, the rest of Moon's team, and Mallow's Steenee helped find a Mirage Berry for Sun to give to Tapu Lele. By asking the local Pokémon of the area, Moon was able to quickly gather a large group of Berries and successfully find the one she was looking for.
After Moon exited Ultra Space, Dartrix and the rest of Moon's team were stolen by Faba and stored at Aether Paradise in Secret Lab B. In Regeneration!! The Power of the Sun and the Moon!, after Moon escaped Faba, she headed to Secret Lab B and found Dartrix and the others safe. While trying to prepare medicine for Gladion, Moon was ambushed by Aether Foundation Employees and had her team hold them off while she finished her work. While battling against a Huntail, Dartrix evolved into Decidueye.
Decidueye is normally laid-back and easygoing, but takes on a more serious personality when battling. He is quite strong, and when first introduced, was shown destroying a part of Professor Kukui's house alongside Popplio. Despite this, he is weaker during the day due to being nocturnal, needing to store up his energy to become active at night. Decidueye has a close bond with Moon, formed on the day they met when she helped him recover from Popplio's Bubble Balloon. The two grew so close in such a short time that Rowlet refused to leave Moon and chased after her when she left for Akala Island.