Dexio and Sina are first met by the player in Route 4 where they discuss about Flabébé and the Fairy type. Dexio then introduces himself as a Pokémon-voyage veteran who received his Pokédex from Professor Sycamore two years beforehand. In the gate to Lumiose City, Dexio tells the player about friendship and gives the player TM27 (Return). After arriving at Lumiose City, Dexio tells the player to follow Sina to Professor Sycamore's lab.
Further on, he and Sina give the player the Coastal Kalos Pokédex at Route 8 and the Mountain Dex at Route 13.
The duo are soon met again at the Kalos Power Plant, disguised as the defenders of Kalos. They arrive to stop Team Flare, but by the time they get there, the player has already driven the villains away. They thank the player for their help and offers their assistance before taking off. Dexio is met alongside Professor Sycamore when the player reaches Dendemille Town to discuss the legend of XerneasX/YveltalY. The player meets them again in their disguises in Lysandre Labs, where they ask if the player has seen a man as tall as a stretched-out Seviper that Team Flare has been searching for. There, they pretend that they have never met the player and warns them to be safe when fighting against Team Flare.
After Lysandre is defeated in the Team Flare Secret HQ, the masked Dexio and Sina thanks the player for their victory. Tierno reveals that he and Trevor teamed up with Dexio and Sina to free the Pokémon Team Flare tied to the stones on Route 10 to power their ultimate weapon. When Tierno states their names, Sina denies their true identities. Dexio also reveals that he and Sina destroyed the machine Lysandre used to spy on peoples' Holo Casters.
After the player has become Champion, Dexio is seen with Sina in the parade Professor Sycamore made for the player and their friends.
Pokémon Sun and Moon
The player first meets Dexio alongside Sina during their first visit to Heahea City. Upon discovering that the player has a Z-Ring, heS/SinaM battles the player. After the battle, Dexio gives the player a Zygarde Cube, and the two of them task the player with finding the Zygarde Cells and Cores scattered across Alola.
The player later meets the duo in an Aether Foundation Labratory on Route 16, where they explain how to use the Zygarde Core to create a Zygarde to add to their team.
After becoming the first Champion of the Alola League, Dexio and Sina meet the player again at Ancient Poni Path. Dexio begins to mention Mega Evolution, and battles the player to demonstrate it, using his Mega Ring to Mega Evolve his Alakazam. After defeating him, Dexio and Sina explain Key Stones and Mega Stones to the player, and give them a Key Stone of their own, suggesting that they attach it to the opposite end of their Z-Ring. Dexio also gives the player Alakazam's Mega Stone, Alakazite.
Dexio appears as a possible opponent and partner in the Battle Tree. In the Battle Tree, Dexio's selection of Pokémon is more balanced, with a focus on Pokémon exclusive to Pokémon Sun.