Oh, Issei assumed Archer was sleeping with Tohsaka. So it really only looked like an older man in a sort-of attractive teenage girl's room alone to him.

...Which wasn't much better.

"I've considered that, but even if he does believe me, I really don't want him to find out about this place. If he were to join, he'd be affected by these things, and..." She sighed again. "I don't want to be the cause of that, even indirectly."