Just as the Umbreon, upgrade an Eevee during Daytime and make sure it's happy with you. (Feed it loads of Vitamins and berries and what not), and a other way without needing Vitamins or berries is to make it hold a Soothe Bell and level it up during the day!
TIP: All you need to do is upgrade while it likes you 1 level, and it's sure to evolve to Espeon.
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The best psychic Pokemon on Pokemon diamond is alakazam. It can beat an infernape level 100. who carez about a dull looking Pokemon with two unusually big mustaches... ew... but I like Lugia and Espeon... and Gallade... and Gardevoir... bronzong all the way haha
Evolve it from eevee
Flareon can't evolve you can only evolve eevee into: /flareon/jolteon/vaporeon/leafeon/glaceon/umbreon/espeon
Well the weakest are umbreon and espeon because there all normal. But if i were to evolve my eevee i would evolve it into a Pokemon type i don't have or just put eevee in with a ditto boy and/or girl. actually umbreon is dark type and espeon is psychic, i would get flareon or jolteon
Ive hear my friends say that in Pokemon yellow you can evolve eevee in a espeon with a moon stone