Alright, first off, the gender, protag options, and core personality are, in my opinion, secondary to the single most important choice. What era are we in? Halo Reach, Halo:CE, Halo 2, and Halo 4 are our options and would you look at that it's a straight line timeline. Everything after Reach can be reached *eventually* but starting later won't let you go mess with things before. While it's likely that most of the core story of the games will stay the same, it's not about the destination, it's about the grunts we kill along the way.
Assuming we manage to not die during the main invasion and that the game is taken as canon, the Pillar of Autumn is one of the last ships off that rock, leaving a nice straight forward way to bridge the gap to Halo 1. If we *fail* to get on the pillar of autumn, the Master Chief brings the Ascendant Justice back to reach and ends up on the surface, giving a second chance to link up with the main plot prior to halo 2 (and, in fact, this is where I would expect for a Noble 6 insertion to relink with the plot; there's plenty of time and material to play cat and mouse with the covenant before the master chief time travels backward in order to link up with his forerunner artifact). So if we want to skip out on stuff, there's plenty of space to skip out on.
From there, it's all about character design. I prefer to go with the protagonist as Noble Six, who is a sufficiently blank slate that you can write anything in there. Master Chief really isn't a blank slate, and I feel that overwriting him has significant story consequences on the whole; I don't feel that *any* spartan in his shoes would have gotten the job done. Chief isn't just "Spartan 117" he's John. So a conditional vote for Canon Replacement; if we end up with a different era, swap it for a vote for In Parallel.
Of the 4 given personality types, I'm actually disinclined toward J&J (for some reason it brings to mind Tucker). Based on having to contrast with Fireteam Noble for the beginning of the story, I'm going to have to do a write in. If Calm and Reserved is the Chief, Sadistic and Dire is definitely something that brings to mind Emile. Professional and Composed would clash with the presentation of Six as a "Lone Wolf" (which, granted, is all Tell rather than Show characterization) type, and so I'll throw in a write in.
[X] Female
[X] Fall of Reach
[X] Noble Six
[X} Deadpan Efficiency: Sworn to defend Earth and all Her colonies, by any means. No act too brutal, no quarter asked or given, the covenant aren't covered by the geneva convention and they do so look good soaked in napalm. They're a little off putting, keeping people at arms reach, but don't let that fool you. Humor dryer than Crassus and sarcasm with more bite than a Brute lurk just under the surface, locked away behind that helmet.
Now that the vote's out of the way, I have to question how meta we're allowed to vote and about base premise, particularly what you as the quest runner want to see out of this quest.