Mairin is a young Pokémon trainer who journeys alongside Alain to master Mega Evolution. She has fair skin, orange eyes, and spiky maroon hair. Her attire consists of a yellow long-sleeved shirt, a light green cloak with darker green edges, long dark green pants with light green triangular markings, brown suspenders, gray socks, and brown shoes. She also wears a yellowish-orange scarf tied with a ribbon, a small bracelet on her right wrist, and a green hat adorned with orange triangles and a yellow pompom.

She exhibits a cheeky and adventurous nature, frequently trying to lift Alain's spirits despite his dismissals. Mairin often misunderstands Mega Evolution, at times thinking the trainers themselves transform instead of their Pokémon. She shows anger when Alain leaves her behind but proves her loyalty by protecting him even at great personal risk. After Alain calls her a burden, she falls into a deep depression, especially when her Chespie becomes ill from a mysterious green glow. Mairin admires both Alain and Serena for their skill and confidence.

Her journey begins after receiving Chespin as a starter Pokémon and witnessing Alain battle a Mega Absol. Fascinated by his ability to Mega Evolve Charizard, she decides to follow him and learn about the phenomenon, ignoring his requests to be left alone. During their travels, she successfully catches a Flabébé with Alain's guidance. Their relationship evolves from a rocky start into one of mutual trust and respect.

In Hoenn, Mairin meets Champion Steven and watches Alain's battle against him, which is interrupted by Lysandre. The group discovers ancient ruins housing a megalith and encounters Rayquaza. Alain is injured protecting Mairin from Rayquaza's Draco Meteor, leaving her fearful of both the legendary Pokémon and the risks Alain takes. She later begs him not to confront Primal Groudon and Primal Kyogre, but when Alain tries to leave her behind for safety, she secretly follows on Steven's airplane. She rescues an unconscious Alain during the clash between the Legendary Pokémon, dragging him to safety with the help of Steven's Metagross.

Back in Kalos, Mairin and Steven contact Professor Sycamore and learn Alain is at Lysandre Labs. She watches him battle multiple Mega Evolution trainers but is deeply hurt when he claims her presence hinders his growth. She runs away crying, and Chespie wanders off alone. Chespie is later found emitting a strange green glow and falls into a coma. Mairin remains devotedly at Chespie's side in Lysandre Labs, where Lysandre promises to find a cure. Alain vows to help heal Chespie and make Mairin smile again.

Mairin appears in Lumiose City during the Kalos League finals, briefly interacting with Clemont's Chespin and explaining Chespie's condition to Ash and his friends. She expresses hope to meet Alain after the finals and is excited to learn he plans to gather Mega Evolution energy to cure Chespie. During Alain's battle with Ash, she cheers for him and is happy to see him smile. After his victory, she becomes concerned by Lysandre's disappearance and later encounters Team Flare grunts. Professor Sycamore rescues her, and they flee together while discussing Lysandre's true intentions. Mairin joins Sycamore and Serena on a mission to retrieve Chespie from Lysandre Labs, traveling by helicopter and facing attacks from Team Flare administrators.