The female Heroic Spirit held out her right hand beside herself, and a horse appeared there. She then lightly mounted it, and vigorously rode it up the ravine.
The cloth wrapped around her arm was still overflowing with concentrated divinity. She circulated the potent magical energy into the horse through its bridle. Horse and rider, moving as a single organism, rapidly threaded their way through the rain of gales. Massive chunks of rubble had begun to fall back to earth. She began to gallop lightly over them, eventually riding across even the boulders still in mid air.
Witnessing the female Heroic Spirit traveling backwards up the waterfall of rubble, Tine was sure.
She can only be Rider!
That would mean that a hero who naturally possessed the qualities of an archer had taken the form of Rider in this War. It might be reasonable to suppose that the force of her bow had been enhanced by the divine aura flowing from the cloth wrapped around her arm.
Then that cloth is a Noble Phantasm... one that strengthens its user’s abilities...
As she watched, the female Heroic Spirit climbed high into the sky, and at last arrived at the peak of the falling rubble. Then, pinpointing the figure of the mystery Archer below her, she drew her bow taut atop her steed.

Fate/Strange Fake: Volume 3, Chapter 7 ("Day 1, Afternoon")