Now that she's speaking more, looking at him, calling him by name, something inside Souji's head clicks. He'd never been made aware of the age changing that Kiseki imposed upon its residents at times, but he can't worry about the reasoning behind that now. He could easily say this is some twin sister of the girl he had helped the other day, but ... he's not so sure.
Even though Souji is keeping his voice calm and level, his sword is still raised despite having no visible threat and he takes a step back when she makes any indication of coming closer. Are those... fangs...? Had she had those before?]
They attacked me, but I don't seem to be hurt. I wanted to make sure you were okay. ... Yuuki-chan.
[He's still not sure it's her, but testing her name is a good way to find out. It's not as though he hasn't dealt with evil versions of his own teammates before.]