Synonyms for Guileless (same or very similar meaning)
WordNet sense 1 (free of deceit):
transparentWordNet sense 2 (characterized by honesty and fairness):
square, straightWordNet sense 3 (free from guile):
honestWordNet sense 4 (simple and natural; without cunning or deceit):
artlessFrom the ODE community, based on WordNetadd/edit