"You're not the only person who knows. But you're the only person who would understand what they know," he says simply, "Forgive me for not knowing how to best broach the subject," he picks the jobstone up and settles it back into a pocket somewhere, "It's not something that I need associated with the 'Hero of Eorzea' for reasons I'm sure you understand." It's partially the darkness aspect, but it's also, as well she knows, that there are things Dark Knights do in the shadows so that people like him can walk in the light. Perhaps keeping it a secret helps the Warrior of Light, but he keeps the secret more to protect the Dark Knight side of the coin, to save himself a place in the shadows.

"Would you care for assistance with your armor?" he's clearly putting the conversation at least somewhat behind him, behind them both, for the moment.