After rescuing the heroes from Team Rocket at Professor Oak's lab, Gary sent Eevee to battle Ash's Pikachu. The latter started off with Quick Attack, but was countered by Eevee's Reflect. Pikachu then fired a Thunderbolt, which Eevee avoided with Double Team. Eevee went to counterattack with Quick Attack, yet Pikachu's Agility allowed it to dodge. Pikachu went to unleash Thunder, but was quickly stopped and defeated by Skull Bash.
Gold and Silver series
Gary, using his Umbreon, battled a Trainer's Alakazam, evading its Zap Cannon attack. As Umbreon was unaffected by the latter's Psychic attack, it evaded its Headbutt and retaliated with Tackle. Alakazem went to use Dynamic Punch, to which Umbreon used Agility to dodge. Alakazam then used Double Team and closed in on Umbreon, who identified the real Alakazam and hit it, then used Quick Attack. However, Alakazam used Teleport to shift itself, and fired a Hyper Beam. Umbreon evaded this attack, and to finish the battle, used Hidden Power to defeat Alakazam.
The heroes met up with Gary, and seeing his battle, Misty and Brock believed Umbreon was as strong as its Trainer. Later, when Gary gave Umbreon food, the city he resided in suffered a blackout. The two helped the heroes in battling Team Rocket, who used a machine to snatch Ash's Pokémon. In order to restore the power to the city, Umbreon used Flash to help others find the part for the generator, and was later used to power that generator up.
Gary's Umbreon was shown on a computer, as Ash went to research which Pokémon Gary could use during the Silver Conference.
Ruby and Sapphire series
To rescue Aerodactyl taken by Team Rocket, Gary sent Umbreon to battle Cassidy's Houndour and Butch's Hitmontop. Umbreon avoided Houndour's Bite and attacked it with Tackle, then evaded Hitmontop's Rolling Kick. Crystal sent Ivysaur, who tackled Houndour and used Vine Vhip on Hitmontop. To defeat Team Rocket, Umbreon used Quick Attack, whereas Ivysaur's Solar Beam blasted the duo away.
Diamond and Pearl series
In Sinnoh, finding Pokémon Hunter J trying to capture a Shieldon, Gary had Umbreon use Sand Attack on her henchmen's Golbat. It was later used to attack J's airship, along with Gary's Electivire and Ash's Pikachu, to rescue Shieldon. When Gary encountered Ash again, he offered to help train his Gligar. Gary sent Umbreon, and just it went to use Shadow Ball, Gligar felt initimidated and flew on Ash's head, canceling its Steel Wing attack.
Encountering Hunter J once more, Gary had Umbreon use Psychic. However, Saturn had Toxicroak use X-Scissors on Umbreon, to prevent their plan from failing.
Umbreon was also used when Gary encountered Team Galactic. Along with Gary's Electivire it challenged Saturn's Toxicroak but Toxicroak was too fast, even for Umbreon, and although it was not knocked out, it was unable to stop Team Galactic.
With Professor Oak