Calem is a boy living in Vaniville Town with his friends Serena, Tierno, Trevor, and Shauna. He recently moved into Vaniville Town. His mom owns a Rhyhorn and Fletchling, the latter of which wakes Calem up at the start of the game.
As a non-playable character
Calem is a young boy who lives in Vaniville Town when the protagonist moves into town. Calem is the main rival of the character, choosing the starter Pokémon which is super effective to the player's. He is also the character who teaches the player and Shauna how to catch a Pokémon, catching a Bunnelby with the use of his Fletchling. Soon after, he can be seen in Santalune Forest with the other friends.
Calem is the child of two prominent Pokémon battlers, and is seen as one of the two best battlers of the group of friends, alongside the protagonist.
The next time Calem battles the player is in front of the Coumarine Gym.
When the group of friends meets up and decides to go to the haunted area in Route 14, Calem is the only one of the friends to not go to the house, claiming that he has to train. He then helps the player defeat Team Flare at the Poké Ball Factory.
The next time Calem battles the player is in front of the Anistar Gym. After the player defeats the gym, he will ask for a rematch, but cancels his challenge after Lysandre announces his plan of eradicating all life forms not a part of Team Flare. He then helps the player defeat Team Flare get to the legendary Pokémon and defeat Team Flare for good.
"Welcome to Vaniville Town. The name's Calem. I live next door."
"The renowned Professor Sycamore lives here in the Kalos region. I heard he has a task for five kids including us. But I'm surprised he knows who you are. You did just move here to Vaniville, after all."
"This is the meeting place, <player>. I'll introduce you. Everyone, this is <player>."
"So, <player>. This is Tierno. He can really tear up the the dance floor... And this is Trevor. He's a little shy, but he always gets perfect scores on his tests..."
"You should decide what we call you."
"Good to meet you, <starter>. My name's Calem. Because of you, I can finally become a real Pokémon Trainer. Thanks!"
"In Kalos, kids are chosen to carry a Pokédex and go on an adventure. But you need to give that letter to your mom before you do any of that!"
"I'm going to go to Santalune City's Gym and to on the Gym Leader. So you know, Pokémon Trainers test their strength by battling with the Gym Leaders in each Pokémon Gym."
"I learned a lot from others. Here, I have something for you guys-- "Adventure Rules." I wrote down 10 different tips every Trainer should know. Here's one for you."
"Look in those rules if you're confused about something."
"So get this, neighbor, Lysandre is the head of Lysandre Labs, which created the Holo Caster, a video-messaging device. You already know about Diantha, right? She is one of the most famous movie stars in the world, after all, I wonder what the two of them are doing here together."
"I'm Calem. It's a pleasure. This..."
"Hmm... I guess Pokémon Trainers all battle each other, no matter who they are. Oh, yeah. I wanted to ask you something, neighbor. How about you and I compete and see who ends up stronger? I kno3 we set off on our journey just after you arrived in Vaniville, but we are neighbors, and having a competition with you sound like fun. I won't lose though. Just so you know.""
"OK, let's battle. Tierno and Trevor, you'll battle against <player's nickname? and me!"
After winning
"Everyone sure has a different approach to being a Trainer. Well, that was fun, but I guess we should head to Ambrette Town."
Ambrette Town
"Hey, neighbor!"
"This is Ambrette Town. You go through the aquarium to get to Cyllage City. But first, we should go to the {{DL|Ambrette Town|Fossil Lab]] and have them tell us about Fossils. Maybe we'll learn something about those mysterious stones related to Mega Evolution."
"So, there are two members of Team Flare this time... Are you ready to battle alongside me, neighbor?
Yes: "I just fought with another one of these Team Flare guys."
No:
"If you're just trying to make yourselves happy, I won't stop you. But when you threaten people, it's like who do you think you are, anyway?
"Is he there?"
"Was Team Flare here?"
"Never mind... Guess he was so absorbed with looking for Fossils he didn't notice."
"I'll take this one, then."
"So, we went looking for the assistant, and we found that suspicious group called Team Flare as well... Still, with those Team Flare people running around, it would be better to train more so I can travel safely. I guess the best thing to do to get tougher is challenge the Cyllage City Gym."
"Some Pokémon evolve through friendship, and others only evolve in certain places"
"Trust... In other words, the bond between Trainer and Pokémon!"
Before battle
"<player>, let's battle for it! No matter what the reason is, a battle is a good chance for us to compete and see who Vaniville Town's best Trainer really is!"
"How about it?
Yes: "I will master Mega Evolution and set myself apart from other Trainers. I'll also do it for my Pokémon--we've come so far together, and they never stopped believing in me, their Trainer. You and your Pokémon are stronger.. I can tell just by looking at you. But I'm not going to lose. The winner of this battle will be me!"
No:
Being defeated
"Your bonds with your Pokémon are really strong. I don't like losing much, but... I'm sure you'll be able to Mega Evolve your Pokémon!"
"I'll bet the Professor will be happy if you master Mega Evolution. That's one way you can show mastery as a Trainer."
The character X is modeled after Calem. When he was young he won a Kalos Junior Tournament. However, the pressure of being a celebrity at a young age had its effect on him and he became a shut-in.
Trivia
Pre-release media referred to Calem as "Xavier" (Japanese: エックスX).