During the H4 reveal at E3 when they show john is ambushed by above. The knight's face? opens and you can see a skull like face .
I'm a total noob at halo facts :(
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I think of them like the Cybermen. Outwardly synthetic, but requires organic components like a heart, a brain and a nervous system. You can see what looks like a set of small organic arms underneath the synthetics weapon arms. So maybe. We will see though.