Sakura and Shirou
"Yes, of course, destroying my familiar in itself is not difficult", she responded, nodding. "But, it looked like a powerful attack, and she summoned it out of nowhere with no apparent effort, not to mention that she managed to detect the familiar in the first place. Plus, she can fly and, apparently, mind-control people to an extent. And she has a huge mana signature, too, far bigger than anything we've ever seen before, and we've seen ancient heroes summoned as familiars made purely of magic."
Sakura decided not to mention that one of said familiars was currently contracted to her. It wasn't important right now, and they didn't know for sure that the girl was friendly, so she didn't wish to give away unnecessary information about their status.
"Yes, she's right", Shirou added. "Her mana signature alone indicates that she must be a powerful being, or at least an extremely unusual one. The attack just confirms it, and her actions indicate that she is likely not friendly. We certainly need more information."
"If you can manage to listen to what she's saying from a distance, that would be very helpful. She's inside a building right now, so we can't see much, but we can get a little closer if we must. We have to be careful, though", he continued.
"Well, I can create another familiar, one which hides its mana signature better. And keep it further away. Whether we'll be able to use it to spy on her properly I'm not sure, though", Sakura said.
"So, what do you think?" she finished, addressing the other girl.