In reality:
— She is very kind at heart.

synonyms: at bottom, deep down, in spite of appearance, inside.

Roget 821: heart and soul, from the bottom of one's heart, ab imo pectore [Lat.], at heart, con amore [It], heartily, devoutly, over head and ears, head over heels.   

Roget 820: in one's heart etc. n.; at heart; heart and soul etc. 821.