I personally think that the concept of core characters to a franchise like Halo is a Bad Idea. The weight of some stories are borne entirely by a main character, granted, and characters like the Stainless Steel Rat or Harry Dresden are absolutely fun to read about. But really, there's only so many stories that can be told with them at the helm. Push it too far and you end up with characters like Luke Skywalker and Superman, who get dragged into conflict after conflict because A) By this time he's been reduced to a bland slice of toast that can be inserted into any story and B) Fan recognition.

I think that a good character should have an arc. They undertake a journey, they change, they grow, and maybe they travel another one. Eventually, in a sprawling universe like Halo or Star Wars, other characters should step up to the plate.

I don't know how much exposure a character should get, but I think it's better for a character to get a good sendoff while there's a few stories they still appear in, as opposed to a character who overstays their welcome and slowly looses that spark that made us love them in the first place.