Robert Reynolds. The Sentry. The Void. All of these in one body. Young Robbie Reynolds drank a concentrated form of the Super Soldier serum, gained the power of "a million exploding suns", he became the Sentry, The Golden Guardian of good.
He also became the Void, the darkness in everyones hearts. For all the good he does as the Sentry, the Void comes to balance it out. And this leads to one key point in the Marvel Universe.
For every Captain America, there is a Red Skull, Wolverine has Sabretooth, The X-men has the brotherhood, and Spider-man has Venom. Similar powers with different psyches beneath. Robert Reynolds, as the Sentry was set to be the most powerful person in the Marvel U. He need a counter, an opposite, and there's only one person in the Marvel U with that kind of power.
Reynolds' mind was split, be it naturally or as suggested the work of a mutant, he became two halves to a whole. A light and a dark. In the first issue of the original mini this duality was hinted at, with Reynold's supposed alcohol problem, and issues with addiction. Reynolds starts as a weak, drunken, overweight, agoraphobic nobody who dreams of being something more. Throughout the mini he becomes the hero he imagines, and then, once the truth of The Void is brought to light, Reynolds does what can be considered the most heroic moment in Marvel History.
He gives it up. Knowing what he was going to be after, he still changes back, to a nobody who dreams of being a hero no one remembers.
But if no one remembers the Sentry, can there still be a Void? Unfortunately, it appears so. On the very first page, Reynolds is awoken, by what he believes is the Void himself, but it is not until Robert drinks the "serum" that the Void is released. This leads me to believe that Robert is always the Void, but the Void can only be free when Robert and everyone else knows who he is. The Void lies deep within him, always on the border of Robert. Without his powers one may believe that the Void may have represented himself as a separate personality inside Reynolds.
House of M 2, which was an alternate reality tale, showed Robert powerless, having never been the Sentry, and still he dreams of the Void, the blackness that overtakes him.
So which is the dominant personality, the Sentry, or the Void. I think that Robert will always BE the Void, only occasionally becoming the Sentry. The Void, who represents the base nature of people, as simplistic, violent, angry creatures, like most other animals, will never go away, just be hidden underneath a golden veneer. Not unlike regular people, we all have a darker, violent, animalistic side, that we hide under socio-cultural themes and customs. The Void, is just these basic emotions, multiplied a million fold.
Best way I can sum it up.
Once more The Sentry, forever more the Void