Play with Hardcore, who came with the latest update. We are not perfectly sure how this mystery character can be unlocked. It's likely that you have to kill the BBQ Grill Boss at the end of a shoot skies "level". Maybe there is a second condition needed. Please tell us about your experience with the unlock! Thx to everyone who helped with suggestions (Steffi, CK1, gadger, Rhino, Ken Ash...)!
Unlock Guide for all Secret Characters in popular mobile games