May received an Egg from a Day-Care worker named Christopher 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. It showed interest in participating in the Wisteria Town Pokémon Contest, but May said it needed more practice first.
Eevee in the Gardenia Town Contest
In Off the Unbeaten Path, May and Eevee competed in the Pokémon Orienteering Contest, with the goal being to find five checkpoints spread across the island. After obtaining all but one of the checkpoints, the two met up with Max and Munchlax, who still needed two more. While discussing it, Jessie and Team Rocket showed up and cut the bridge they were standing on. Ash and Brock showed up just in time to save them, after which they were approached by Swellow, who informed them Team Rocket was up to no good. Due to this, the group ended up abandoning the race.
Eevee made its stage debut in What I Did for Love!, in which it went up against Brock and his Marshtomp in the Gardenia Town Pokémon Contest. Although Marshtomp could use its fins to sense Eevee's underground movements and detect exactly where it would come up after using Dig, Eevee eventually scored a win for May.
Eevee took part in the Battle Round of the Kanto Grand Festival in Thinning the Hoard!. 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.
In Once More With Reeling!, Eevee performed in the Appeals Round of the Terracotta Contest, an informal event not sanctioned by the Pokémon Activities Committee. It proved a great performer, coming in first for the appeals and allowing May to compete in the Battle Round.
In A Full Course Tag Battle!, May introduced her newly evolved Glaceon soon after she was reunited with the group. She revealed that Eevee evolved into Glaceon after coming into contact with the Ice Rock near Snowpoint City. Dawn commented that Glaceon was very cute. It was called out once again 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 in its first battle as a Glaceon. Glaceon got a head start by hitting Piplup with Secret Power. Because it was used indoors, Piplup was paralyzed. Piplup shook it off by getting into the water and the two engaged in a fierce battle of moves. It ultimately ended with Piplup using Bubble Beam and Glaceon reflecting it with Mirror Coat. Both Pokémon were hit by the water from the Whirlpool as time ran out. Piplup ended up winning by points, securing the Aqua Ribbon for Dawn. Although the loss saddened Glaceon, May said that it did great.
In Partners in Time!, Glaceon watched Ash's Masters Eight Tournament match against Leon. During the battle's climax, Glaceon was shown to be anxious alongside Serena's Sylveon.
Glaceon touching the Ice Rock that makes it evolve
As an Eevee, Glaceon had the typical traits of its species: playful demeanor, affection for its Trainer, and loyalty. It was also quite energetic. Being a recently hatched Pokémon at the time, Eevee had great curiosity as shown by its fascination for the ocean.
Upon evolution, its personality changed moderately. While Glaceon retained its loyalty and affection for its Trainer, its playful demeanor had been replaced by a more assertive and slightly aggressive one that shows mostly during battle.