Mairin is a young Pokémon trainer from the Pokémon anime, distinguished by her short stature, fair skin, orange eyes, and spiky maroon hair. She wears a yellow long-sleeved shirt, a light green cloak with darker edges, long dark green pants featuring light green triangles, brown suspenders, gray socks, and brown shoes. Her accessories include a yellowish-orange scarf, a small bracelet, and a green hat decorated with orange triangles and a yellow pompom.

Her personality is cheeky and adventurous, driving her to seek new experiences. She persistently tries to cheer her travel companion, Alain, despite his frequent dismissals. Confusion sometimes arises, like her mistaken belief she could personally Mega Evolve instead of her Pokémon. Mairin shows fierce loyalty, protecting Alain in dangerous situations even at personal risk. She experiences significant emotional distress after Alain states she burdens him, compounded when her Chespin, Chespie, falls ill following exposure to a mysterious green light. Mairin also expresses admiration for Alain and Serena, citing their skills and confidence as inspirations.

Her journey starts upon receiving Chespin as her starter Pokémon. Witnessing Alain battle a Mega Absol, she becomes fascinated by his ability to Mega Evolve Charizard and resolves to join him to learn the phenomenon. Despite Alain's initial reluctance, she follows him across regions. During their travels, Alain advises her on capturing a Flabébé, which she successfully adds to her team. Their relationship evolves from contentious to mutually respectful, especially after Alain shields her from a Draco Meteor attack by Mega Rayquaza during an investigation of ancient ruins, sustaining injuries himself. This incident fuels Mairin's fear of Legendary Pokémon and concern for Alain's safety.

In Hoenn, Mairin pleads with Alain to avoid confronting Primal Groudon and Primal Kyogre. When he tries to leave her behind for safety, she stows away on Steven Stone's aircraft to follow. Seeing Alain struggle against the Legendary Pokémon, she rushes to his aid after he falls unconscious, helping Steven's Metagross move him to safety. Their reunion is cut short by Lysandre stealing the megalith, prompting Alain to abandon her again at Devon Corporation. Undeterred, Mairin travels to Kalos with Steven.

At Lysandre Labs in Kalos, Mairin watches Alain battle ten Mega Evolution trainers. Between matches, Alain states traveling with her hinders his growth. Emotionally devastated, Mairin retreats; Chespie wanders into a restricted lab. There, Chespie encounters small green creatures under experimentation; during an explosion, Chespie shields one creature and absorbs its glowing energy. This exposure causes Chespie to collapse into a coma. Lysandre vows to find a cure, motivating Alain to complete Lysandre's Mega Evolution energy collection project.

During the Kalos League finals, Mairin resides at Lysandre Labs, monitoring Chespie's comatose state and sending Alain video updates about minor movements in its spikes. She briefly encounters Ash and his friends in Lumiose City, returning Clemont's lost Chespin. Discussing her own Chespin's condition brings her to tears, though she expresses hope for its recovery at Lysandre Labs. Lysandre later tells her Alain's efforts in the League will gather energy to cure Chespie. Mairin attends Alain's battle against Ash, initially overjoyed at his victory but concerned when Lysandre vanishes after the win.

Following Lysandre's public declaration to reshape the world destructively, Mairin questions his true intentions regarding Chespie's treatment. Confronted by a Team Flare grunt, she is rescued by Professor Sycamore and his Garchomp. She joins Sycamore and Serena to retrieve Chespie from Lysandre Labs, fleeing with them amidst attacks from Team Flare admins. During the escape, she watches Serena's battle prowess with admiration before continuing toward the laboratory via helicopter.