A "mischievous streak" refers to a tendency or inclination for someone to behave playfully or mischievously, usually in a harmless or joking manner. Full definition
After a series of pot smoking and underage drinking episodes, Prince Harry's mischievous streak landed him in a London rehabilitation center for a day. (businessinsider.com)
Playing a wise - cracking, too - smart - for - her - own - good straight - A high school student with a mile - wide mischievous streak, Stone, 21, gets to show off both her comic timing and her dramatic chops. (nola.com)