Mairin's starter Pokémon is a Chespin that she nicknamed Chespie (Japanese: ハリさん Hari-san). Chespie remains outside its Poké Ball. It is helpful as it rescues Mairin when she falls over and helped her catch her Flabébé.
Chespie reappeared in Mega Evolution Special II where it was accompanying Mairin in Hoenn.
It reappeared in Mega Evolution Special III. It accompanied Mairin whilst she snuck on board a helicopter as Alain forbid her from coming. It protected Mairin from ice shards caused by the rampage of Primal Groudon and Primal Kyogre who was trying to get a knocked out Alain to safety.
It reappeared in Mega Evolution Special IV, in which it accompanied Mairin to Lysandre Labs. After Alain refused to allow Mairin to travel with him, she asked Chespie to give her a moment alone. This resulted in Chespie following two scientists to a room where it found a Zygarde Core and some cells being subjected to some sort of evidently painful experiment. Chespie rescued the Zygarde after the equipment overloaded, but the energy Zygarde had been subjected to caused Chespie to fall into a coma, devastating Mairin.
Chespie's known moves are Vine Whip, Pin Missile, and Toxic.