WordNet sense 1 (not by design or artifice; unforced and impromptu):

unstudied, uncontrived

WordNet sense 2 (openly straightforward and direct without reserve or secretiveness):

heart-to-heart, open

WordNet sense 3 (characterized by an inability to mask your feelings; not devious):

artless, ingenuous

WordNet sense 4 (characterized by directness in manner or speech; without subtlety or evasion):

blunt, forthright, frank, outspoken, plainspoken, point-blank, free-spoken, straight-from-the-shoulder