An ordinary looking, seemingly average girl/guy you kind of know through friends or work. They hang out, seem amicable, but never really stand out in any particular way. You might even talk shit about how boring they are... until you find out they're a world class Spanish guitar player, NASA engineer, capoeira instructor and trendy nightclub owner, all while dating a Swedish supermodel.
"Hey, have you met Dave?"
"Dude, Dave sucks. The guy is like wallpaper."
"Yesterday he bicycle-kicked a beehive into a KKK meeting."
"Yeah, he's a secret badass."