We were told the Master Chief never removes his helmet. It was a sacred rule, a pillar of the games' storytelling. Halo Season 1, Episode 2 – "Unbound" – shatters that pillar not with a bang, but with a quiet, terrifying exhale.
When Cortana says, "You're broken, John," she doesn't mean physically. She means his conditioning – the very thing that made him the UNSC's greatest asset – is cracking. The visions of his childhood self on Eridanus II aren't flashbacks. They're a rebellion. For the first time, the Spartan isn't hunting an enemy; he's hunting a memory of who he might have been. Halo Temporada 1 - Episodio 2
Yes. He takes it off. Twice.