May's Blaziken is May's starter pokémon.
In Japanese, Blaziken was voiced by Chinami Nishimura (Rikako Aikawa briefly) as Torchic and Combusken. Katsuyuki Konishi voiced it as a Blaziken. It's English voice actor is Bill Rogers, who also voiced it as Combusken in TPCi's dub. In English as Torchic it was voiced by Rachael Lillis, in the 4Kids dub Darren Dunstan voiced it as Combusken. He will become Mega Blaziken in Pokemon X and Y.
In Get the Show on the Road!, Blaziken was given to May by Professor Birch in Littleroot Town when he was still a Torchic. She chose it as her starter Pokémon because she thought Treecko was creepy and scary (though she later called them cute in Trees a Crowd), while the starter Mudkip had attacked her earlier with Water Gun to the face. However, she seemed to like Torchic's appearance and sweet nature. Early on in the journey, Torchic was very inexperienced and ditzy. It did not know how to battle, and tended to wander around obliviously; it also tended to panic when in dangerous situations. On the group's first encounter with Jessie's Seviper (still wild at the time), the Poison-type frightened Torchic by trying to eat him.
When May started to ignore Torchic in favor of Beautifly, Torchic's personality began to change: he grew more determined to improve himself and recapture May's attention. His competitive streak continued to grow as it lost most of its childishness and confusion; this culminated in Torchic's evolution into Combusken during A Shroomish Skirmish. Combusken itself had a period of being highly aggressive, but this seems to have passed and its personality seems to have evened out more now that it apparently feels that it no longer has to overcompensate. Combusken became May's strongest Pokemon, proving to be very capable in battle as well as in contests.
In Once More with Reeling, during the break of a Pokémon Contest, Ash and co. were trying to stop Team Rocket from stealing Pikachu and other Pokémon. In the midst of the ensuing battle, Combusken evolved into Blaziken, and used its Blaze Kick to send Team Rocket flying, upon which Ash remarked "would I love to battle you". Afterwards, May used Blaziken in a battle against Ash's Sceptile. Although it had a type advantage over Sceptile and its Blaze ability activated at the same time as Sceptile's Overgrow did, the battle ended in a tie. Later, he left for Hoenn with the rest of May's Pokémon, along with May herself and Max.
Blaziken appeared in A Full Course Tag Battle!. He was used in the Seven Stars Restaurant tag battles, in which it battled alongside Dawn's Piplup. When it was first called out, Ash seemed to be pleased at seeing it again, and Brock remarked that it had gotten stronger as well.