Trevor's Charmeleon is the evolved form of
Trevor's Charmander. It evolved sometime after the Summer Camp and it was instrumental in helping
Trevor defeat
Viola to get the Bug Badge. It's personality did not change as a result of evolving into Charmeleon.
Ash Ketchum and friends hear it in the bushes and he realized that it was in a battling mood so he asked
Trevor to battle.
Ash's Fletchinder was sent out to battle it to help cool down its temperament at the time. The two traded blows until
Ash's Fletchinder was able to finish it off with a powerful Steel Wing. Despite loosing the battle,
Trevor's Charmeleon felt refreshed after the battle.