When Platinum set off on her journey to Mt. Coronet, she took Ponyta from her mansion with her.
Rapidash first appeared as a Ponyta in Stagestruck Starly, where he was used as Platinum's steed. He was mainly used as her mode of transportation, not seen in many battles until A Conk on Cranidos's Cranium, where he participated in Platinum's first Gym battle against Roark and his Onix. The battle ended quickly, however, as he was defeated by a combination of Double-Edge and Slam that sent him flying into a wall and was knocked out as a result.
In To and Fro Froslass, Rapidash participated in Platinum's Gym battle against Candice where he battled her Froslass. Rapidash initially held his own due to his fast speed, but was recalled after suffering several hits and replaced with Lopunny. After Lopunny sacrificed herself with Healing Wish, Rapidash returned to battle, fully healed, allowing him to defeat Candice's Froslass, earning Platinum her seventh Gym Badge.
During the battle against Jupiter in Stopping Sableye, he helped alongside Platinum's other Pokémon as well as Candice and Maylene's. Despite their best efforts, the trio were unable to stop Jupiter from capturing Uxie and were defeated, After Jupiter left, Platinum took the unconscious Candice and Maylene to the Snowpoint Temple to rest. Platinum had Rapidash use his flames to keep them warm while they were trapped in a blizzard.
In Exit Empoleon, Rapidash fought in Platinum's Battle Arcade challenge. During the battle against Dahlia, Platinum and Dahlia had to switch Pokémon due to the roulette, meaning Rapidash and Platinum's other Pokémon were forced to fight against their Trainer. Although both sides were reluctant at first, both Platinum and her Pokémon manage to fight at their full strength, which eventually ended with Platinum as the victor.
Rapidash is a modest and quiet Pokémon, preferring to stay off to the side unless needed. Rapidash has good battling skills, even though he often loses the battles he participates in. He is very loyal and had an extremely strong relationship with Platinum, and is willing to do anything to help her out.