labile
A total chameleon who can switch personalities faster than you can say 'I’m not a bad person, I just have a bad day.'
My mom’s labile, one minute she’s baking cookies, next minute she’s yelling at the mailman.
That guy in class is labile, he went from crying to laughing in ten seconds.
My brother’s labile, he’ll be happy about the game, then mad because I won.