Rider
Because you're Sakura, Rider thought immediately as she heard the question, asked in a very familiar voice.
Rider knew, of course, that it wasn't really true. This girl wasn't actually Sakura. True, she looked and sounded almost identical, and there were far too many similarities for it to be a pure co-incidence. But, where Sakura's training had involved being raped by her grandfather's worm familiars, the only "worm" involved in Oka's training was a cute, fluffy thing that she obviously loved dearly. Her childhood may not necessarily have been perfect, but it was certainly far, far better than Sakura's had been.
Yet, despite the fundamental differences, right now, Oka was frightened, sad and lonely, and Rider couldn't help but feel immense sympathy for her. She knew she wasn't Sakura, but she looked and sounded like Sakura and, right now, she was frightened and lost. She needed someone to help look after and comfort her, and Rider knew she wouldn't forgive herself if she left the poor girl alone.
Of course, she knew she couldn't tell the girl this. "You look like my master" was not a very good reason for protecting and looking after someone. But, ultimately, it was a bit more than that. It wasn't just that she looked like Sakura, she acted like Sakura too. Or, at least, like Sakura might have if she'd been able to have a normal childhood. And, just like Sakura, she seemed to be a sweet and loving girl who Rider would be happy to be friends with.
Still, Rider was unsure how to respond. How best could she explain herself without sounding weird or creepy, and scaring the girl away? She pondered her response for a while, waiting to ensure that the two people who had been annoying Oka were not returning, before turning to the girl with a gentle smile on your face.
"Because you seem like someone I would like to get to know better, and you were frightened. I have the strength to protect you, so why should I leave you feeling unsafe?" she said.
It sounded corny, almost like something Shirou might say, but Rider really couldn't think of any better way to explain it. She wanted the girl to be safe, and she wanted to make friends with her, and she couldn't do that if the girl was too frightened to speak.