Shirou
Shirou was now thoroughly confused, as the look on his face would show. He knew Rider was dead, he'd even watched her die, and that wasn't even in this world. How could she possibly be here? At the same time, though, he saw no reason for the girl to be lying, and she seemed nice enough, as was demonstrated by the concern she was showing for his health, albeit unnecessary, so he didn't want to be mean to her.
"I'm fine, but thanks for the concern", he said, politely.
"I definitely saw Rider die, though. I don't know how she could be here.", he continued, pausing for a moment to consider the possibilities. "Hmm...."
He knew, from Rin, about the Second Magic and the possibility of parallel worlds, where things were different. Maybe a different Rider had been summoned into some other world, and made her way here somehow. Or, maybe, even the same Rider. After all, they had met people who were definitely not from their world here already, not to mention the girl in front of him, who could have easily been Sakura's twin, why could there not be people from worlds which were somewhat more similar?
"Well, Rider is a class name, so perhaps she's not the same person", he added. "Or she could just be from a parallel world, I suppose."
Shirou started to think deeply, wondering what this world was like. The girl seemed to associate Rider with the name "Sakura", somehow, so it was a good guess that the two were connected, but he had no idea how, or why she had survived.
"Hmm, did this 'Rider' know someone called Sakura, then?" he asked, curious.