Rono debuted as an Aron in the end of Distracting Dustox, as one of the two Pokémon Sapphire started her journey with.
In Lombre Larceny, Rono was used to help rescue Mr. Stone's Castform from a water fountain. With his ability to chew through metal, Rono was able to eat enough of the fountain enough for Sapphire to rip it off and rescue the Castform.
During Sapphire's Gym battle against Roxanne, Rono was used to battle the Gym Leader's Nosepass. Noticing that Rono was unable to defeat Nosepass, Sapphire attempted to switch with Chic, but couldn't due to Nosepass's Magnet Pull. Eventually, Sapphire has Rono defeat Nosepass by turning away from north and making it lose its balance.
After undergoing intensive training for Sapphire's second Gym battle in the Granite Cave, Rono evolved into Lairon. Later, he was used against Brawly's Makuhita. Despite his newly-gained strength, Makuhita quickly countered his moments, defeating Rono and almost knocking him out.
During Sapphire's training battle against Winona, Rono, along with Pilo and Dono, was used to battle Winona's Pokémon. Together, the three easily beat Winona's team, earning Sapphire the Feather Badge.
During Sapphire and Ruby's training on Mirage Island, Rono became stronger and evolved into his final form, Aggron.
In the Emerald arc, Rono was used against Tucker's Salamence in the Battle Dome. The battle went unseen, but Sapphire ended up the loser of the fight.