“No need to thank me. I really am poking my nose in where I’m not wanted. If you thank me just for acting on my whims, it might encourage me to be even more officious. I know; if you’re tired of walking, why don’t I fetch you a horse? I’m sure William will produce one if I spend a bit of magical energy...”
“No, I’m fine, really!” Ayaka remonstrated with the Heroic Spirit. She did not know what he might do if left to his own devices. Then she voiced a question that had been on her mind for some time.
“Wait, who’s William? If you don’t want to tell me anything, why do you keep throwing out names of people I don’t know?”
Saber looked away for a moment, then responded with a disarming laugh.
“Oh, it’s just that you were looking so glum and lonely. I thought it might cheer you up if I hinted that we have lots of invisible friends, so...”
“That’s just creepy. Stop it.”
“Understood; I’ll stop. It doesn’t really matter; I’ll tell you about ‘everybody’ in good time.”

Fate/Strange Fake: Volume 3, Chapter 8 ("Day 1, Afternoon")