Regirock first appeared in Battling the Enemy Within!, where it defeated Ash's Sceptile, despite its type disadvantage and the fact that Ash was possessed by the King of Pokélantis. Regirock was also used to battle Reggie's Drapion and won. It is Brandon's most used Pokémon and is also the first Regi added to his team. Regirock is arguably the strongest of Brandon's Pokémon because it has had more victories on screen than any of Brandon's other Pokémon.
Brandon later used Regirock again in A Pyramiding Rage where it battled Paul's Pokémon in a 6 on 6 battle. Regirock defeated Paul's Hariyama, Electabuzz, Ursaring and Magmar while Regice and Registeel dealt with Lairon and Nidoking.
Regirock was used once more in Pillars of Friendship to stop an enraged Regigigas from going into a rampage. After Regirock and the other Regis saved Regigigas from J, Brandon decided to stay with Regirock, Regice and Registeel at Snowpoint Temple to protect Regigigas from any future attacks.