The O.N.I. Spartan
Finally we have the mystery guy from the image 343 released on Friday. 343 confirmed two things about this character, first that he is male, second that this is a new character. He looks like he is wearing an all black Spartan IV armor and he is rocking the O.N.I logo on the left side of his chest plate. Whoever he is, he has to play a significant role in Halo 5, not only is he on the cover, but he's on top and the Chief is on the bottom.
It is almost like he is the oppisite of the Master Chief, a reflection by the way the picture looks. Perhaps the O.N.I. Spartan is a clone of the Master Chief. Hell, he might actually BE the Master Chief. Maybe the Chief leaves the UNSC proper after Cortana died to join O.N.I. and this is his new armor. Wouldn't that be a plot twist? Other than that we don't know much, below are a closer look at him, followed by a picture of the O.N.I. logo.