Trust. It isn’t easily given, nor is it easily accepted.
He was the first person she trusted in Eorzea. She doesn’t know how or why, but she did. She took that risk. In the years that followed, there was a time when their friendship had disintegrated to such a degree that it was a stab to the gut, and all they knew was how to twist the knife deeper. Even then—even through that—she trusted him. Trusted him when they were both broken and stupid and fuelled by spite and hate and grief.
The faith she has in him is a different kind now. Stronger, implicit, as natural as drawing breath. When they are in the thick of it, he is her guardian and she his shield. She trusts him. Even when he is being impetuous and stubborn and a fool. Even when he has no regard for his own well-being. When sacrifice is a word that comes terrifyingly easily to his mind.
She knows he will do the right thing. And she will be there, to pick him up when he is broken, just as he has done for her. Come what may, she will be there.