Mairin is a young, energetic, and notably clumsy girl who begins her journey as a novice Pokémon Trainer in the Kalos region. She is distinguished by her spiky maroon hair, orange eyes, and a somewhat tomboyish outfit consisting of a yellow long-sleeved shirt, a small light green cloak, long dark green pants held up by brown suspenders, a yellowish-orange scarf, and a green hat with a small yellow pompom. Her primary partner is a Chespin she nicknames Chespie, who almost always stays outside of its Poké Ball and accompanies her on her adventures. She also catches a Flabébé, which she nicknames Bébé, early in her travels.

Mairin's personality is defined by her adventurous spirit and boundless curiosity, especially regarding the phenomenon of Mega Evolution. Despite being a beginner, she possesses a forceful and persistent attitude, often refusing to take no for an answer. This trait is most evident when she decides to follow the experienced trainer Alain after witnessing his Charizard undergo Mega Evolution, even though he repeatedly tries to leave her behind for her own safety. Her eagerness to learn sometimes leads to comical misunderstandings, such as when she briefly expresses a desire to learn how to Mega Evolve herself, rather than having her Pokémon do so. Her clumsiness is a prominent quirk; she frequently trips or falls, and her loyal Chespin is often quick to help her back up using its vines.

Her primary motivation throughout the story is a deep-seated care for the well-being of those close to her, particularly Alain and Chespie. She worries about Alain’s safety when he throws himself into dangerous battles, questioning why he takes such risks. Her journey takes an emotional and tragic turn when Chespie, after helping a Zygarde Core escape from Lysandre Labs, is exposed to a strange energy and falls into a mysterious coma. This event devastates Mairin, causing her to pause her own training and remain by her Pokémon’s side, sinking into despair. Her desperate hope for Chespie’s recovery is manipulated by the villain Lysandre, who falsely promises that the Mega Evolution energy he is collecting will be the cure.

Mairin’s role in the story is deeply intertwined with Alain’s character arc. She serves as his moral anchor and a reminder of his connections to others, which contrasts with his single-minded pursuit of power. Her relationship with Alain evolves from him seeing her as an annoying burden to him genuinely caring for her, even if he expresses it through harsh words meant to protect her by pushing her away. She also forms a bond with Professor Sycamore, who acts as a caring guardian figure, and she becomes friends with Ash and his companions after meeting them during the Lumiose Conference. Her interactions with Serena, in particular, show her admiration for other trainers' confidence and skills.

Throughout the series, Mairin undergoes significant development. She begins as a carefree and clumsy tag-along but is forced to confront deep emotional pain and despair following Chespie’s coma. Her character is tested during the Team Flare arc, where she realizes Lysandre’s deception and bravely participates in the mission to rescue her Chespin from the Giant Rock. Her resilience shines through as she continues to believe in Chespie’s recovery and Alain’s inherent goodness. By the story’s conclusion, after Chespie is finally saved and wakes up, a relieved and happy Mairin chooses to continue traveling with Alain, having matured from a naive newcomer into a more determined and emotionally strong individual. Her notable abilities as a trainer are still developing, but she successfully captures a Flabébé using a strategic approach suggested by Alain and demonstrates a deep, empathetic bond with her own Pokémon.