Synonyms for Ingenuous (same or very similar meaning)
WordNet sense 1 (lacking in sophistication or worldliness):
innocentWordNet sense 2 (marked by or showing unaffected simplicity and lack of guile or worldly experience):
naif, naiveWordNet sense 3 (openly straightforward and direct without reserve or secretiveness):
candid, openWordNet sense 4 (characterized by an inability to mask your feelings; not devious):
artlessFrom the ODE community, based on WordNetadd/edit