The Harry Potter actor posted a selfie in 2016, where these two poked fun at his "XI" tattoo. "A lot of people ask what my tattoo means. Now you know. @milliebobby_brown #XI #Eleven," he wrote. If you've seen Netflix's hit, you know Millie plays a badass character named Eleven, who audience members couldn't help but fall in love with. It looks like Matthew is a card-carrying member of the Stranger Things fan club just like the rest of us.