Mairin is a young Pokémon Trainer from the Kalos region with fair skin, orange eyes, and spiky maroon hair. She wears a yellow long-sleeved shirt, a light green cloak with darker edges, dark green pants featuring light green accents, brown suspenders, gray socks, and brown shoes. Her accessories include an orange-yellow scarf tied with a ribbon, a bracelet on her right wrist, and a green hat decorated with orange triangles and a yellow pompom. She starts her journey after Professor Sycamore gives her a Chespin, which she nicknames Chespie. Observing Alain battle a Mega Absol sparks her interest, leading her to insist on joining his quest to battle all known Mega-Evolved Pokémon, overcoming his initial reluctance. Guided by Alain, she captures a Flabébé, showcasing her developing skills. Mairin displays a persistent and adventurous spirit, often trying to cheer up the dismissive Alain. Her concern for his safety deepens during encounters with Legendary Pokémon Rayquaza, Primal Groudon, and Primal Kyogre in Hoenn. When Alain shields her from Rayquaza's Draco Meteor and is injured, she conquers her fear to rescue him from Primal Groudon and Kyogre's clash, aided by Steven Stone's Metagross. After Alain abandons her in Hoenn to protect her, she travels to Kalos with Steven to find him. At Lysandre Labs, she witnesses Alain battling ten consecutive Mega Evolution Trainers. Distraught by his harsh rejection where he blames her for hindering his strength, she flees in tears. During her distress, Chespie wanders into a restricted lab, encounters Zygarde Core cells, and rescues one Core by helping it escape through a vent during an energy experiment explosion. This act exposes Chespie to unknown energy, rendering it comatose. Devoted to Chespie's recovery, Mairin remains at its side. She stays temporarily at Lysandre Labs where Lysandre promises a cure, then travels to Lumiose City, confiding in Ash Ketchum's companions about Chespie. She attends the Kalos League finals to support Alain, expressing joy at his victory. However, she grows suspicious of Lysandre after he reveals his plan to destroy the world. When Team Flare grunts attempt to capture her, Professor Sycamore and his Garchomp intervene. She later joins Serena and Sycamore to retrieve Chespie from Lysandre Labs amidst Team Flare's attacks. Mairin's journey shifts her resolve from seeking adventure to prioritizing Chespie's recovery and questioning authority, her experiences hardening her determination and culminating in active defiance against Lysandre's schemes.