If you thought you were the biggest Stranger Things fan out there, prepare to relinquish your crown to Matthew Lewis. The Harry Potter actor posted a selfie with the sci-fi show's breakout star Millie Bobby Brown on Instagram on Sunday, where the 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, whom audience members couldn't help but fall in love with. Apparently, Matthew — who played Neville Longbottom in the Harry Potter franchise — is a card-carrying member of the Stranger Things fan club just like the rest of us. While we wait for the show's second season, we'll go ahead and add that to our long list of reasons to adore him.