His commander. The only person he fully trusts at the moment. He shows his transparent self for the first time.
Not only to her, but to us, the people watching the show.
Clones are bred with a sole purpose, told they are expendable, and if they die fighting, most wouldn't care.
They are human, but they aren't treated as such.
Hidden beneath their masks there's people. That's what we see when she removes his helmet. She's telling him its okay to feel how he is doing at the moment.
And this is the breaking point for many. Rex is hurting, he's mourning, His brothers, the other soldiers, even himself have been used for all this time. Manipulated into fighting a war that was lost from the start.
All the deaths, all the used soldiers, brainwashed... It hurts him as it hurts everyone else. Even more as they are his family.