Kanto Battle Frontier
May received an Egg from a day-care worker in May's Egg-Cellent Adventure, which hatched into an Eevee in Time Warp Heals All Wounds. Since then, it became a major part of May's team, watching every one of May's Contests since it hatched.
Eevee made its Contest debut in What I Did for Love, in which it went up against Brock and his Marshtomp. Although Marshtomp, being part Ground-type, could detect where Eevee was when it used Dig, Eevee eventually scored a win for May.
Eevee also took part in the Kanto Grand Festival Battle Round in Thinning the Hoard!, in which it showed great battling skills and impressive synchronicity with May's Munchlax, being able to combine its attacks with Munchlax's to take down Harley's Cacturne and Wigglytuff.
Eevee appeared again in Once More With Reeling! to perform in the Appeals Round of the unofficial Terracotta Town Contest, where it proved to be a great performer, coming in first for the appeals and allowing May to compete in the Battle Round.
Sinnoh
In A Full Course Tag Battle!, May introduced her newly evolved Pokémon soon after she got reunited with the gang. She revealed that her Eevee evolved into a Glaceon by coming into contact with the Ice Rock near Snowpoint City. It was called out once again in order to help out Roman and Kylie in retrieving the restaurant's stolen food, where it battled alongside Dawn's Buneary. Both of them started off with a double Ice Beam, causing James's Carnivine and Jessie's Seviper to freeze. Followed by its Iron Tail and Buneary's Bounce, they blasted off their opponents.
In Strategy With a Smile!, May used Glaceon in the finals of the Wallace Cup, where it went up against Dawn's Piplup. The battle concluded with Dawn as the winner, though Glaceon lost the match by only a few points. During this time, Dawn herself (when she first saw it) commented that Glaceon was very cute.