Prior to the start of the Sun & Moon series, Umbreon was an Eevee that Gladion found injured near his house. Gladion ran the injured Pokémon back to his house and asked Hobbes for help, who rushed it to a Pokémon Center. It later appeared in one of Lillie's flashbacks where it was helping the family out in Lusamine's garden, and again when it appeared when Lillie was still a baby.
After accompanying Gladion when he confronted Ash in Mission: Total Recall!, they returned to their hotel room. They were confronted by Faba, who demanded that Gladion return Type: Null, which Gladion refused to do. Alongside Lycanroc, Umbreon battled Faba's Hypno and Alakazam, only to be attacked by Miracle Eye, allowing Alakazam to knock it out with Psychic.
In Faba's Revenge!, Umbreon helped Ash rescue Nebby from Faba, only to watch in horror as a Nihilego emerged from the Ultra Hole and merged with Lusamine, who sacrificed herself to save Gladion and Lillie, and return through the Ultra Hole.
In Rescuing the Unwilling!, Gladion used Umbreon, Lycanroc, and Silvally to fight Lusamine's Absol, which was controlled by Nihilego, to allow Ash to save Lusamine. Duing the fight Gladion revealed that Absol was Umbreon's sparring partner. Umbreon deflected Absol's Razor Wind attack with Dark Pulse, before being trapped alongside Lycanroc by Absol's Mean Look. In 10,000,000 Reasons to Fight!, Umbreon and Lycanroc were released from the Mean Look after Silvally used Multi-Attack on Absol, allowing them to continue fighting. When Nihilego had Absol get back up again after Gladion appeared to defeat it, Gladion told Umbreon and Lycanroc to take care of Absol before riding off on Silvally to get to Lusamine.
In SM127, Gladion went searching for his father's Zoroark on Melemele Island, with Umbreon joining him. During the search, it battled a Mandibuzz that attacked them, easily defeating it. At night, when Umbreon came across what appeared to be a Lurantis, Gladion ordered Umbreon to attack the Bloom Sickle Pokémon, causing it to revert into Zoroark. Gladion urged it to come with him and help him find his father together, which it agreed to.
Whenever it is not battling, Umbreon is always resting, as seen in A Masked Warning!, where it was lying down on the grass, relaxing whilst watching Nebby sleep. It has a strong bond with its Trainer, helped by the way he found it injured and helped it back to full health.
Umbreon is close friends with Lusamine's Absol. In Rescuing the Unwilling!, Gladion revealed that it used to be Umbreon's sparring partner. When Absol was under the control of Nihilego, it battled Absol. Once Nihilego was defeated and its control over Absol disappeared, Umbreon affectionately rubbed cheeks with Absol.
Despite its caring personality, it is also very eager to fight, as shown in SM127, where, despite sending a Mandibuzz that it battled flying away, it still wanted to keep fighting, forcing Gladion to command it to forget about it. In the same episode, it was also very observant of its surroundings and was able to tell if a Pokémon was real or an illusion that a Zoroark had transformed into.
In battle, Umbreon is shown to be a strong Pokémon, as seen in SM131, where it was able to withstand a Subzero Slammer.