While in the northern areas of Sinnoh, Hareta is put to a "test" by the Snowpoint CityGym Leader, Candice. Hareta was reluctant because he wanted to just go fight Team Galactic, but Candice insisted that unless he passes this test, he will have no chance at all of defeating them. She sends out her three Pokémon, Regirock, Regice and Registeel, explaining that the "test" is to defeat two of them in a Double Battle. During the battle, Hareta's Riolu evolves into Lucario and defeats Regice and Regirock. Having passed the "test", Candice determines that Hareta is capable enough to be taken to try to catch Regigigas at the Snowpoint Temple.
After being awoken by the other three Legendary giants, Regigigas is off to a Slow Start, but soon takes out all three Regis in a single strike. Candice wonders whether she may have been a bit reckless in allowing something this powerful to be awoken, but Hareta replies that he'll be fine. During the battle, Regigigas seems almost impervious to anything thrown at it; however, Hareta continues on. Eventually, Hareta realizes that Regigigas isn't the viciously dangerous monster it's made out to be; rather, it's just like him. All it really wants are fierce battles. However, due to its immense power, any time it got serious, everyone else became afraid of it. Instead, it came here to sleep where it wouldn't harm others. Hareta invites it to come along with him to help in the fierce battles ahead, water beginning to trickle into the room as Piplup (who was earlier smashed through a wall with Hareta) returns with a large wave which everyone rides back to the exit.
Regigigas and Hareta
Regigigas activating Slow Start
After its capture, Regigigas isn't seen again until it competes in the battle against Mitsumi at Team Galactic HQ, its Slow Start disposed of thanks to Kadabra's Skill Swap, allowing it to move as full speed from the start. It is one of Hareta's only two Pokémon to last the entire battle, helping to finish it up with the "Piplup and Regigigas Hang Ten Surf". After this, it was finally unable to battle any longer.
At the Pokémon League, Hareta sent it out during his battle against Flint after his Empoleon lost to Flint's Rapidash. Hareta also sent out Misdreavus in order for Regigigas to not activate Slow Start but the referee deemed it as illegal, so Hareta saved Regigigas for later. After Misdreavus and Drifblim drew, Hareta sent out Regigigas against Flint's Rapidash. Flint made the starting move to finish Regigigas off easily in order to take advantage of its Slow Start Ability. However, Regigigas did not get defeated as Hareta told it to hang on. Regigigas landed the finishing move on Rapidash with Mega Punch. Despite Regigigas being Hareta's last Pokémon, Flint forfeited the match as his Steelix wouldn't have handled Regigigas. Regigigas was the second Pokémon sent out against Koya where it was able to easily defeat his Metagross. However, when Koya sent out Mamoswine, Regigigas was knocked out in one blow by Mamoswine's Take Down.
Later, Regigigas battled a Giratina when it was about to attack Hareta and the others. Regigigas began with a Mega Punch but it had no effect on Giratina since Hareta didn't know that Giratina was part Ghost. Instead, Regigigas used Ice Punch on it which worked. Regigigas then worked with Hareta's two rivals in order to continue battling it.
Personality and characteristics
Regigigas immediately became attached to Hareta, as shown when Hareta was not hurt by its punching attack. During battle, it has shown to utilize athleticism. For example, it jumped very high and held onto the ceiling when Hareta battled Mitsumi while she was a member of Team Galactic.