His eyes snap to her face, and he just looks - confused. She's never caught him off guard like this before. He doesn't even get to ask why, so startled by it that he just kind of stares at her at a loss for a long few moments, which probably gives her the time to keep going.

It doesn't help like he thinks, because it's an incredibly kind thing to say and he doesn't know what to do with it. It's hard for him to believe it. I'm glad it's you is not really a sentence people use towards him, not for something so good.

Cassian looks away because looking at her becomes too much. He still thinks Rey and Jyn both deserve better than him, but: "Thank you." It's so quiet he's not even sure he said it, half a question because he's still baffled. "I would never let anything happen to her."

It's easier to focus on that element of it, on Jyn trusting Rey is safe with him, because at least that part he knows to be true.