This Sylveon is Kandalee Brown's second Pokémon she caught on her journey prior to the Eevee Series, and Kandalee's strongest Pokémon so far. She is the mother of Nancy's Sylveon.
Sylveon started out as an Eevee, who evolved into a Sylveon in Eevee and Larvitar's Big Adventure!. She evolved by thinking a lot about Kandalee and protecting William, Jacob, Kandalee, etc.