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This came to mind just recently, but what if the reason we haven't seen Killer's eyes in his SBS child shot or his whole face as an adult, is because he's related to the Three-Eye Tribe, which, in my speculation, is an endangered group? We haven't seen much of them aside from the half-blooded Charlotte Pudding, and if they can read the Poneglyph language very well with the Voice of All Things, then I wouldn't be surprised if the World Government made an order long ago to make they never get the chance to read those blocks. They did the same to Ohara, after all. Granted this is just speculation, but when and if the spotlight shines on Killer, I think it would be an interesting plot twist.