Pre-series
Gible being found by Alain
In Garchomp's Mega Bond!, it was revealed that Garchomp, as a Gible, rampaged through Lumiose City, only to be stopped by Sycamore's assistant Alain. Alain took her to Sycamore's laboratory, where she then came to live. She gradually started to trust Sycamore and the other Pokémon at the lab, and after some time evolved into a Gabite and finally into Garchomp, becoming the main test subject as he researched Mega Evolution.
Garchomp made her debut in Lumiose City Pursuit!, when Team Rocket attempted to capture Froakie, and defended it from Team Rocket's device which then attached itself to her. This caused her severe pain resulting in her going on a rampage through Lumiose City and heading to the top of Prism Tower.
Garchomp and Professor Sycamore
Once there, she was carefully approached by Ash who refused to allow Pikachu to use any moves against her since she couldn't help her behavior. Soon, with Ash and Froakie's help, the device was removed and Garchomp returned to her old self.
In Mega Evolution Special III, Garchomp was by Professor Sycamore's side whilst there was a TV report about Alain's battle with Mega Rayquaza, Primal Groudon, and Primal Kyogre.
In Garchomp's Mega Bond!, Garchomp Mega Evolved into a Mega Garchomp thanks to a Garchompite that he found and a Key Stone that Meyer lent to him as Blaziken Mask.
Garchomp was sent out to defend Mairin from a Team Flare Grunt's Skorupi in A Towering Takeover!. She later Mega Evolved and joined the fight against Team Flare in Coming Apart at the Dreams!.
In Curtain Up! Fight the Fights!, Garchomp watched Alain's Masters Eight Tournament match against Leon.
As a Gible, she was a hostile and frightened Pokémon. Gible caused trouble at the lab until Professor Sycamore reached out to her. The Professor stayed calm, even after Gible bit him in defense, and she became relaxed in his company. As Gible evolved, she grew more relaxed and more socialable, interacting with Alain and the other Pokémon at the lab.
By the time Ash first met her in Lumiose City Pursuit!, Garchomp is shown to be a very gentle Pokémon, despite her fearsome appearance. She immediately showed her concern for Froakie and seemed to recognize the urgency of the matter. She enjoys company, and happily allows others to pet her. Garchomp and Professor Sycamore's bond was shown to be very strong in Garchomp's Mega Bond! when they achieved Mega Evolution.
