Clemont's Chespin was initially one of the three Pokémon a Trainer receives from
Professor Sycamore when they start out on their journey. It took the basket of macarons that
Serena had baked the night before and began stuffing itself and didn't want to share them.
Sophie noted that Harimaron loves pranks.
Team Rocket captured
Professor Sycamore,
Serena and
Bonnie but Harimaron was able to follow a macaron crumb trail to their hideout. It was initially more concerned about getting the macarons back but it finally realized that Team Rocket was also a threat, so it helped out and manged to stop the Mega Mega Nyarth mecha.
During its time with
Clemont in the crisis situation, the two began to bond and just as
Clemont was leaving
Professor Sycamore lab, it indicated that it wanted to travel with
Clemont.
Professor Sycamore gave
Clemont Harimaron's monster ball and he got it.