Trevor appears in Pokémon X and Y as a friend of the player. Trevor delivers the player his or her Pokédex on behalf of Professor Sycamore, and he aims to be the first among his friends to finish collecting data for his Pokédex. Trevor and his older sister live in Lumiose City, near Jaune Plaza. Ever since their parents left them behind in order to travel around the world and train their Pokémon, it has been Trevor's dream to travel all over Kalos with a Pokédex of his own. He first battles the player on Route 7 alongside Tierno. From then on, Trevor engages in his own kind of battle with the player, comparing how much each has completed the Pokédex.
When the Abomasnow in Frost Cavern is threatened by Team Flare, Trevor comes and helps the player by battling one Team Flare Grunt's Mightyena to let the player beat Mable and save the Abomasnow.
"What?! You want me to nickname someone I just met?! Shauna, you shouldn't put people on the spot like this. Well...how about something low key? Maybe...something like Big <player's name's first letter>/<player's name's first letter>-kins..."
"Uh, pardon me, but I have something for you from the professor as well. I have something that will help you understand Pokémon on a much deeper level."
"Um, so you see... The Pokédex I just gave you is a high-tech device that automatically records the Pokémon you encounter! The reason you get one is because the professor wants us to go on a journey with our Pokémon and complete the Pokédex. To put it another way, it's an important mission from the professor. I'm sure of it!"
"Why, look for Pokémon, of course! The professor did ask us to complete the Pokédex, after all. And furthermore, different Pokémon prefer living in different places. To put it another way, it's a chance to find different Pokémon than the ones in the forest!"
If talked to
"Each Pokémon move has PP... That stands for Power Points. If the number of points for a move is zero, you can't use that move. Go to a Pokémon Center, and they'll restore them for free."
"Oh, Tierno, do you ever think about anything besides dancing?"
"Good to see you, <player>... Excuse me, I meant <player's nickname>. It really does seem as though Tierno can only think about dancing and moves. When a horde of Pokémon popped out, he just stood there watching them..."
"That's indeed a perfect dream for you, Tierno. I guess I should show just as much dedication to my dream of completing the Pokédex, then! It's good to be serious about some things!"
"Oh, uh, <player's nickname>? Do take this Honey, won't you?"
"You see, using Honey will encourage Pokémon hordes to appear. Though if you are going to challenge a whole horde… You may want to use moves that can hit multiple targets."
"<player>, do you know about this building? It's called the Battle Chateau, and it's famous for being a place where Trainers gather to improve their skills. If I learn even more about Pokémon, I'm sure I'll be able to defeat Kalos's eight Gym Leaders."
"There is a Pokémon I saw when I was little that I don't remember very well. I wonder if filling up my Pokédex will help me remember what it was."
"Hello, <player>! Let's compare our Coastal Kalos Pokédexes and see who has seen more kinds of Pokémon! To put it another way, I'm challenging you to my own kind of Pokémon battle. So, you've seen <number of seen Pokémon in the player's Coastal Kalos Pokédex> kinds of Pokémon..."
If 38 or less Pokémon are marked in the player's Pokédex as "seen":"I've seen 39, so I guess I win!"
If 39 Pokémon are marked in the player's Pokédex as "seen":" We've both seen 39 kinds of Pokémon... Well, I may not have won, but I didn't lose either. In other words, it's a tie!"
If 40 or more Pokémon are marked in the player's Pokédex as "seen":"I lost! How frustrating!"
"Oh! So you've seen Kangaskhan, too, <player's nickname>? Just knowing about the same Pokémon as someone else can sure make you happy!"
"Oh yes, I just remembered. The professor told us to say hello to the Mega Evolution Guru when we reached Shalour City..."
"A person called the Mega Evolution guru."
"I heard he's in that imposing building deep in the city--the Tower of Mastery."
"Hello, <player's nickname>! You arrived very quickly!"
"We were just comparing how complete our Pokédexes are. I won, of course!"
"<player's nickname>! Let's compare how many Pokémon we've seen, just like we always do! This time let's compare how many Pokémon we've seen in the Mountain Kalos Pokédex! To put it another way, I'm challenging you to my own kind of Pokémon battle. So, you've seen <number of seen Pokémon in the player's Mountain Kalos Pokédex> kinds of Pokémon..."
If 5 or less Pokémon are marked in the player's Pokédex as "seen":"I've seen 6, so I guess I win!"
If 6 Pokémon are marked in the player's Pokédex as "seen":"We've both seen 6 kinds of Pokémon. Well, I may not have won, but I didn't lose either. In other words, it's a tie!"
If 7 or more Pokémon are marked in the player's Pokédex as "seen":"I lost! How frustrating!"
"Oh! So you've seen Gible, too, <player's nickname>? Just knowing about the same Pokémon as someone else can sure make you happy!"
"Um... Is the Professor gone already? I was hoping to have him have a look at my Pokédex in person. Oh well, I can still get my Pokédex evaluated at a Pokémon Center's PC... Anyway, I'm heading off to go look for Pokémon in Frost Cavern and fill up more of my Pokédex!"
"It's really cold in here, isn't it, <player's nickname>? Is there something going on up ahead?'
"Hey, you Poké Ball thieves! What are you doing here?!"
"But why is Team Flare amassing Pokémon, energy, and money? To put it another way, why is Team Flare always trying to take everything for itself?"
"<player's nickname>... Did they say only Team Flare will survive? These people have a few screws loose..."
"Um.. If I'm facing Mightyena I should be using..."
"Doesn't battling with Team Flare scare you, <player's nickname>? I'll admit it scares me. I don't like battling them... But we managed to protect Abomasnow by pitching in there. I know I'm the reserved type, and I'm OK with that, but it's important to be bold when necessary. Mamoswine doesn't have to worry about Abomasnow anymore, and that's what matters. Alls well that ends well!"
"Why, the same thing as you, Shauna. We heard about <player> from the professor, and we came running as fast as we could."
"Tierno… It's great when Pokémon Trainers like us can understand the hearts and minds of other Trainers and their Pokémon through battle. But before we do anything else, we should heal your Pokémon."
"Now it's your turn, <player's nickname>."
"I think I will follow the crowd and be your opponent as well. But this time it won't be about the Pokédex. It will be a Pokémon battle!"
"You're even stronger than I thought!"
After being defeated
"There is still so much I don't know. Do I just not have what it takes?"
"Indeed! Well, there's something we must attend to. Be seeing you!"
Geosenge Town (post-game)
When talked to
"Are you wondering about the ultimate weapon, too, <player>? That man wanted to bring his Pokémon back even if it meant breaking the laws of nature. I wonder if I would feel that way if I lost my partner... This is changing the subject, but... Have you caught <Pokémon> yet?"
If the player hasn't caught the Pokémon mentioned
"Hmm. I see that you've seen it, though."
If the player has caught the Pokémon mentioned
"I thought you might have! I caught one, too! It makes you smile when you catch the same Pokémon a friend does, huh? According to Professor Sycamore's research, <Pokémon> has a potential Mega Evolution!"
If talked to again
"According to Professor Sycamore's research, <Pokémon> has a potential Mega Evolution!"