User blog comment:B214/Question & Answers 5/@comment-82.199.205.164-20160228125212/@comment-1359322-20160228152429

She didn't, she first found out only when Issei used it.

"Buchou looks at my left arm. I think she’s looking at my gauntlet.

“……Red Dragon. Until recently there wasn’t a mark like this…… I see, so that’s what it is……”

Is it me or does Buchou's eyes look like as if she's a bit shocked?

“I found out the main reason why Ise was able to beat a Fallen Angel.”

Buchou says it quietly.

“Fallen Angel Raynare. This boy, Hyoudou Issei’s Sacred Gear isn’t a regular Sacred Gear. That’s why you lost.”

Raynare has a puzzled face after hearing Buchou.

“—[Boosted Gear], a Sacred Gear that is said to be the rarest of the rare. The Red Dragon mark on the gauntlet is the evidence. Even you have heard of that name before, right?”"

- Volume 1, Life 4