Board Thread:Questions and Answers/@comment-76.102.30.110-20130715013213/@comment-192.160.117.141-20140408190856

Yes, we know from one of the tapes that Skull Face's extensive scarring occured when he was a child, and the event he talks about closely resembles one that happened during World War II (see Mgsnerdswe's post above). He could only be a returning character if he's also a master of disguise (though, this is actually possible - Kojima likened him to Clayface).