He belonged to Evelyn, a rich Eevee breeder in Sinnoh. Though, this Eevee was not interested in living a life with someone only coveting him because "he's a wonderful Eeveelution": he preferred sleeping when trainers came and chose Eevees for their teams. Irene eventually met the lonely Eevee wandering at night and gave him hope that there are some people that don't care about that kind of things... and in the end he decided to join her team.
As a creature who loves the night, Armaros became an Umbreon; he has the secret hope in his heart to meet a Cresselia, the beautiful goddess of the moon.