Setsuna was the kind of guy who went through hundreds of internal processes. Once the surprise wore off, his mind tried to wrap around what he just saw. His expression shifted back to his usual stoic face while he evaluated the situation.

Eventually, he came to a conclusion, making a thoughtful noise as he did. Whatever one he came to caused his face to shift into another expression he never normally wears.

...was he... smiling? He just might be. If you look closely.

"Christina Sierra..." It was all in the inflection, but that was about as close to Hey, welcome back, I've missed you and I'm glad you're not dead anymore alive, again, as he was ever going to say out loud.