May was born in Littleroot Town, Hoenn to her mother and father Professor Birch. She was a dainty young girl who didn't like battling and had no aspirations to become a Trainer. However, when she was eight years old, she journeyed to Johto with her father who was on a job. She met a young boy whom she played with happily while there. However, an incident between her, the young boy and a wild Salamence resulted in her deciding to become stronger, and, her aspiration to become a strong Trainer began.
When she turned sixteen, her father allowed her to go on small field missions to help him with his research. One day, she was startled by some ruckus above her, which led to her meeting Brendan after he fell from a small cliff. When he woke, they returned to Littleroot Town, Brendan was introduced as the son of the Petalburg CityGym Leader. Upon Brendan leaving, May told her father that she wanted to go on a journey now, becoming a strong Trainer and also help her father by filling up her Pokédex. He agreed and May went on her journey.
Soon after this, May ran into Brendan again and asked for a battle, which he declined. He explained why, and she realized that they had met before as she was the girl that he played with as a child. Upon discovering this, she suggested they head to Petalburg City together, resulting in the two journeying together.
After an incident at Mt. Chimney, May soon found herself in competition with Calem, who was also revealed to be taking part in Gym Battles. This inspired her to continue her hardest during her battles and training, heading off to Petalburg City with Brawly for her next Gym Battle against Norman. During the battle, her Taillow evolved into Swellow.
Initially, May voiced her decision to leave the group once they arrived in Fortree City, due to feeling she wasn't strong enough to help with the threat against Team Aqua and Team Magma. However, after her battle with Winona, she was given advice from the Gym Leader telling her if she believes she's not strong enough she won't be, and instead decided to remain with Lexi, despite the fact that Brendan had left himself with Wallace.
From that point on May became a much tougher opponent in battle, and her Combusken evolved into Blaziken, and her father Professor Birch met with her in Lilycove City where he presented her with her own Mega Bracelet.
May was given Combusken as a Torchic as her Starter Pokémon by her father to help her do field research. May then decided to take Torchic with her to go on her adventure. When May battled against Wattson in Lights On! May vs. Wattson!, Torchic evolved.
Swellow is a Pokémon captured by May. On the way to Rustboro City, May decided she needed a new member for her team for the battle, capturing a Taillow and quickly trained with it for the battle. Taillow continued to perform well for her as it showed aspirations to evolve, and did so when May battled against Norman.
Aron is a Pokémon captured by May. In an incident in Granite Cave, Aron battled against May's Torchic, before showing its willingness to join May. It was then used in her Gym Battle against Aron and continued to be used in further battles.
Plusle is a Pokémon captured by May. When the group were on their way to Mauville City, Plusle and his best friend Minun were being held captive by several members of Team Aqua. After saving the two, May and Brendan caught Plusle and Minun respectively. Plusle prefers not to battle but is completely willing if the situation is called for it, otherwise he prefers to sit at the side and cheer on their friends.