Never, EVER piss of anyone that's frowning scarily in any way, shape or form.
Especially if they're a girl.
Take Erza from Fairy Tail as an example. She has a lot of pissed faces.
But even moreso than angry girls, Never anger a person who is calm/grinning 99.99% of the time, because they become satan himself once angered. Take for example, Sebastian. The world will end the day he goes insane with rage.