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.
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.
The player later meets the duo in an Aether Foundation Laboratory 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, the player meets Dexio and Sina 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.
The player first meets Dexio alongside Sina during their first visit to Heahea City. Upon discovering that the player has a Z-Ring, heUS/SinaUM battles the player. After the battle, Dexio gives the player a Full Restore.
After becoming the first Champion of the Alola League, the player meets Dexio and Sino again at Poni Plains. 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.
The player later meets the duo in an Aether Foundation Laboratory on Route 16 after capturing Zygarde at Resolution Cave. They explain that the Zygarde the player encountered is the 50% Forme of Zygarde and that they had found a 10% Zygarde, which they give to the player. After defeating Dexio, they will explain about the Zygarde Cube and how to use it to combine a Zygarde from its 100 Cells and Cores.
Dexio also appears as a possible opponent and partner in the Battle Tree.
"Ha! Just kidding, Trainer! We weren't really waiting for you. We were on our way to the Power Plant at Professor Sycamore's request. But the gate connecting Lumiose City to Route 13 has been shut by a mysterious power outage. We have to investigate."
"I'm just going to give you the answer. It's Mountain Kalos. So I'm going to take this opportunity and power up your Pokédex again."
"The Mountain Kalos Pokédex has been added to your Pokédex menu. Just tap it to check it out, OK?"
"We're looking for a really tall guy... And I mean tall. If you stretched out a Seviper and stood it up, he'd be that tall. We received some intelligence that Team Flare is also trying to find him for some reason."
"Uh... Anyway... We came here--at a certain professor's request--to stop Team Flare! I assume your goal is the same? Don't do anything reckless, OK?"
If talked to again
"The professor's feeling pretty down, but he's using his network of acquaintances to try to stop Team Flare's plan."
"That's what I would expect from a Trainer doing the island challenge. I felt the bond between you and your Pokémon!"
"You and your Pokémon are the ultimate team! No doubt about it! This battle reminded me of us in the past...and that group of five young Trainers. I've made up my mind! I'll entrust this important item to you!"
If the player collected all Cells and Cores: "Zygarde at 100%... You might be able to control it fully!"
If the player has not collected all Cells and Cores: "If you collect all Cells and Cores, please visit the Aether Base on Route 16 on Ula'ula Island. Use the Reassembly Unit there, and you can transform them into a Zygarde!"
"But we are Trainers. Rather than explaining it all in words, why don't we just show you what Mega Evolution is!"
Upon being defeated
"My Pokémon each gave it their best."
After being defeated
"The Trainer holds the Key Stone! The Pokémon holds the Mega Stone! And that's how you can make your Pokémon Mega Evolve during battle!"
"If you have a Key Stone, you'll be able to make your Pokémon that hold a Mega Stone Mega Evolve!"
"This is a Mega Stone that makes Alakazam Mega Evolve. It's called an Alakazite! I'll give the Key Stone and an Alakazite to you!"
In What Does Charmander Do When It Dozes?, Dexio and Sina were sent to find Professor Sycamore's missing Charmander, which had escaped from his lab. They found Charmander with X and his friends. Mistaking the children for thieves, Dexio and Sina took Charmander, but were stopped by Tierno, who used his Roller Skates to take Charmander back. After Tierno explained that they were merely returning Charmander to its proper owner, Dexio and Sina excused themselves and left.
In Zygarde Appears, Dexio and Sina infiltrated Lysandre Café. There, they defeated a disguised Team Flare Grunt and interrogated him for information on Team Flare's whereabouts. After getting what they wanted, Dexio and Sina reported to Professor Sycamore that Team Flare went into hiding at the Pokémon Village.
In Epilogue, after Team Flare's defeat, Professor Sycamore had Dexio and Sina sent to Alola to investigate the Zygarde Cells that have been spotted around the region.
In PASM16, Dexio and Sina arrived on Ula'ula Island. In the five months that passed since they arrived in Alola, the duo managed to gather sixty percent of Zygarde's Cells. Later, they spotted Sun using StoutlandSearch and decided to ask him to help find the remaining Cells.
In PASM18, Dexio and Sina infiltrated the Aether House, where they met Sun and requested that he help them find Zygarde's Cells. While he was initially reluctant, Sun agreed after Dexio and Sina offered to pay him a large amount of money for each Cell he found. After receiving a Zygarde Cube to store the Cells in as well as a MechaFletchling to keep in touch with Dexio and Sina, Sun set off.
Six months later, Dexio and Sina introduced Samson Oak to the Reassembly Unit at the Aether Base on Route 16, which they planned to use to control Zygarde's forms.