Feel like the author is gonna pull some "oh Rikka secretly knew the whole time" shit. That is, if the author even remembers she still exists because she hasn't gotten more than like 5 panels in 15 chapters I swear.
Rather than erotic, I just think this whole thing is hilarious cause I can't stop thinking of Zepar being Caster and using her crystal ball to spy on the horny shenanigans like she did with Saber.
I wonder why Rikka is left out that much. Even at the "Sensei arc" only Usui was important and now at the summer camp, she gets cast away again. Thought she's the main girl but actually she is the one with the least progression right now.