Mairin begins her journey as a novice Pokémon trainer in the Kalos region after receiving her starter Pokémon, Chespin, from Professor Sycamore. She affectionately nicknames it "Chespie" and usually keeps it outside its Poké Ball. Chespie frequently assists Mairin when she stumbles or faces minor accidents due to her clumsy nature.

Her path crosses with Alain during his mission to battle Mega-Evolved Pokémon. She first sees him fighting Astrid's Mega Absol, witnessing Alain's Charizard undergo Mega Evolution. Intrigued by this phenomenon, Mairin decides to follow Alain to learn more, viewing him as a role model despite his initial reluctance and dismissive attitude. She persists in accompanying him. During their travels, with tactical advice from Alain, Mairin captures a Yellow Flower Flabébé, nicknaming it "Bébé."

Mairin travels with Alain to the Hoenn region. There, they meet Hoenn Champion Steven Stone. She observes Alain's battle against Steven's Shiny Metagross, capable of Mega Evolution. The battle is interrupted by Lysandre, Alain's employer, who reveals an ancient slab hinting at Mega Evolution's origins. The group investigates a temple where Alain and Steven activate a mechanism with their Key Stones, revealing a path to a giant rock believed to be the source of Mega Evolution energy. The disturbance attracts Rayquaza, which Mega Evolves without a trainer and attacks. Alain protects Mairin during the chaos, sustaining an injury. Lysandre then directs the group to Rustboro City to track energy signals linked to the giant rock.

Later, at Lysandre Labs, Mairin attempts to support Alain during his battles against multiple Mega-Evolved Pokémon trainers. Alain harshly rejects her presence, claiming she weakens him. Devastated, Mairin retreats and asks to be alone, causing Chespie to wander off. Chespie encounters an overloaded experiment involving a Zygarde Core and absorbs a surge of strange energy while rescuing the Core. This exposure causes Chespie to fall into a deep coma. Mairin discovers Chespie in critical condition, leading to profound distress. She pleads with Lysandre for help, clinging to his promise to find a cure. Witnessing her anguish, Alain vows to gather enough Mega Evolution energy to heal Chespie.

Mairin remains at Lysandre Labs, constantly watching over Chespie's comatose body. She sends video messages to Alain, expressing hope whenever Chespie shows minor physical reactions. During the Kalos League finals, she visits Lumiose City with Lysandre. Encountering Clemont's lost Chespin triggers an emotional episode as she recalls her own Chespie. She explains Chespie's condition to Ash and his friends but expresses optimism about its recovery. Mairin also reveals her intention to reunite with Alain after the League concludes.

She watches Alain's victory in the League finals alongside Lysandre. Lysandre disappears immediately after, and Mairin witnesses his broadcast declaring plans to reshape the world through destruction. Realizing Lysandre deceived her about curing Chespie, she questions his motives. Fleeing the stadium, she encounters Team Flare grunts attempting to capture her but is rescued by Professor Sycamore and his Garchomp. Mairin informs Sycamore about Ash and Alain's location and joins efforts to uncover Lysandre's true plans. She later accompanies Sycamore and Serena to retrieve Chespie from Lysandre Labs, facing attacks from Team Flare admins during the mission.

Mairin has fair skin, light brown or orange eyes, and spiky maroon or red hair. She wears a yellow long-sleeved shirt, a light green cloak with darker edges, dark green pants secured by brown suspenders, gray socks, and brown shoes. She accessorizes with an orange-yellow scarf, a small wrist bracelet, and a green hat featuring orange triangles and a yellow pompom.

Her known Pokémon are Chespie, which knows Vine Whip, Pin Missile, and Toxic, and Bébé, which knows Magical Leaf.