He asks about other ways to advance, and Fallen Angels!

"So... are there other ways to advance?" Issei asked Rias curiously.

"What, don't want to do the contracts?" Rias questioned, raising an eyebrow.

"No, it's not that!" he shook his head at that. "I'm simply trying to understand more about the world I'm in. I really want to excel, and want to do whatever I can to advance, contracts included, with the other options in mind so I don't miss opportunities!"

"...fair enough," Rias smiled. "Well, there's also military accomplishments, and Rating Games, though those are both more rare than just doing contracts." She stretched as she explained, just showcasing her bare body, much to Issei's delight. "Rating Games are basically combat competitions between Peerages..."

"Makes sense..." Issei nodded, before pausing. "And military accomplishments? Who exactly are we fighting?"

"Angels and Fallen Angels, actually," Rias replied. "Though open conflict between the factions is rare. Angels hate both Devils and their Fallen brethren, but Fallen Angels still don't like Devils. But all three factions are close enough to equal that there's a deadlock, and no one wants to risk fighting one side and rendering themselves **** to the other."

"...tell that to the two that attacked me," Issei grumbled, and Akeno laughed.

"You're just unlucky in that regard. The first one, that pretended to be your girlfriend to kill you, did so while you were human due to your Sacred Gear. You weren't part of any of the factions, but you had potential power that could have been a threat if you became a Devil... which she inadvertently caused, ironically enough. As for the second one, well, Rias-sama had yet to explain things to you, and you were wandering around, so you were assumed to be a Stray Devil by the Fallen Angel that Rias-sama drove away," she explained. "He backed off the moment he realized that pursuing the fight would have consequences."

"Right, what's a Stray Devil?" Issei nodded along, and Koneko face palmed.

"It's practically in the name," she groaned.

"I mean, I can infer a lot, but I want to be sure we're on the same page," Issei replied defensively.

"Stray Devils are Reincarnated Devils that abandon their Peerages for their own selfish desires. They do things like attack people, or otherwise risk exposing the supernatural to the general public," Akeno explained. "So they often need to be put to **** before things can get out of hand. As such, Fallen Angels killing one is not a provocation to us, so for one as apparently unhinged as the one that attacked you, you were an easy target for his bloodlust."

"Until I stepped in and reminded him to stay the hell off my turf," Rias nodded. "Any further questions, Issei?"

He considered for a moment, before he shook his head. "No, I think I'm good."