an iron hand in a velvet glove
A person who has a gentle, sweet, or unassuming appearance or disposition, but who in reality is particularly severe, forceful, or uncompromising. Tom is in for it now with his wife. She might seem like a nice lady to us, but she's an iron hand in a velvet glove when they're at home. The new leader of the country rose to power by promises of democracy and equality to its citizens, but, as his despotic intentions came to light, he soon proved an iron hand in a velvet glove.