Faens are a faction comprised of elves, fairies, and nature lovers and centaurs. They are "strong" against Neanders, and "weak" against Mortiis. Their ability, Miasma, let's them increase their ATK when attacking a Neander creature.
Faens are notorious for being heavy healers, and heavy magic users. While a Faen creature's ATK is not usually very impressive, they normally make up for it with magic abilities which deal heavy damage, and some can even lockdown your creatures. This makes them very good against Frost Armor decks.
Although they are "strong" against Neanders, it isn't recommended that you use a Faen deck against a Neander deck, since most Neander decks have Spiky Bits. Also, even though Faens are great magic users, their abilities become nearly useless when going up against an Immunity heavy deck. Their healing can also be negated by the Rip Ability.