She is a person whose words and heart are one - she never lies.
Tips
culture
Two senses: anatomical (the spot just below the sternum - what English calls solar plexus) and figurative ('mouth' as the outer voice of the 'heart'). The set phrase 心口如一 ('mouth and heart are one') praises sincerity; 心口不一 means saying one thing but thinking another.
memory
心 (heart) + 口 (mouth) - the two endpoints of speech. Match them up for sincerity, separate them for hypocrisy.
