Well they've said it isn't Palmer nor Cortana, and that's it's a new male character. Darn, cause I was gonna say Halsey.\

Now my guess is on some long gone AWOL Spartan from earlier days who's been taken in by the Forerunners and it turns out the UNSC are the baddies and now full out war against them ensues and Halsey teams up with the Chief to blow Palmer's head off because no one likes her. Except 343i...so I guess that means we don't get to kill her. Damn.