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!"
Dexio debuts in the X & Y chapter. He, along with Sina, is first seen when Professor Sycamore asked them to make a copy of the footage of Vaniville Town's destruction so that Lysandre may study it.
Later, they are tasked with finding Professor Sycamore's missing Charmander, which had escaped from his lab. They find Charmander in the custody of X's friends, who they mistake for having stolen it. Wearing disguises, Dexio and Sina reveal themselves and immediately take Charmander away from X's group. Angered, Tierno uses his Roller Skates in conjunction with his dancing skills to quickly take Charmander back. After Tierno explains that they were returning Charmander to its owner, Dexio and Sina excuse themselves and leave.
After Sycamore learns that Lysandre is the leader of Team Flare, he has Dexio and Sina spy on Lysandre Café. Later, Dexio and Sina manage to defeat a disguised Team Flare Grunt and interrogate him for information. They report to Professor Sycamore that Team Flare went into hiding at the Pokémon Village.
After Team Flare is defeated, Professor Sycamore has Dexio and Sina sent to Alola to investigate the mysterious green creatures that have been spotted around the region.
In the Sun, Moon, Ultra Sun & Ultra Moon chapter, Dexio and Sina are first seen arriving at Ula'ula Island. In the five months that have passed since being sent to Alola, the duo have successfully gathered sixty percent of Zygarde's Cells and Cores. Later, Dexio and Sina spot Sun passing by while using StoutlandSearch and wonder if they could ask him to assist them in finding the remaining Cells and Cores.
Later, Dexio and Sina travel to the Aether House, where they are attacked by the Headmaster Oranguru, who believed them to be intruders. After making their way inside, they introduce themselves to Sun and tell him about their request to find the Cells and Cores. While Sun is initially reluctant to assist, he agrees after they offer to pay him one hundred thousand yen for each Cell and Core found. Before Sun sets off, he receives a Zygarde Cube to store the Cells inside as well as a MechaFletchling to keep in touch with Dexio and Sina.