Same word as naive:

aboveboard, artless, callow, candid, confiding, countrified, credulous, forthright, frank, fresh, green, guileless, gullible, harmless, ignorant, impulsive, ingenuous, innocent, innocuous, instinctive, jejune, lamb, like a babe in the woods, natural, open, original, patsy, plain, simple, simple-minded, sincere, spontaneous, square, sucker, unaffected, unjaded, unpretentious, unschooled, unsophisticated, unsuspecting, unsuspicious, untaught, unworldly, virgin, wide-eyed