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Synonyms for Ingenuous (same or very similar meaning)

WordNet sense 1 (lacking in sophistication or worldliness):

WordNet sense 2 (marked by or showing unaffected simplicity and lack of guile or worldly experience):
naif, naive

WordNet sense 3 (openly straightforward and direct without reserve or secretiveness):
candid, open

WordNet sense 4 (characterized by an inability to mask your feelings; not devious):

From the ODE community, based on WordNetadd/edit