[Kase] tried dating other girls but it never worked out.
I was wondering about this, and I think the story is kind of ambiguous about it. I’m pretty sure we can reject the rumours that Kase was a player—that’s the kind of thing that always seems to follow the “cool prince” types in yuri manga. And we know that the supposed affair with Green Drops Senpai was untrue.
But when Yamada says that Kase has more experience, she doesn’t really deny it (unlike the wonderful Girl Friends idiots, who are shocked to find out that they’re both virgins). The “girls shy away when they see my body” (and who can blame them?) statement is also a little ambiguous—some of the nakedness is P.E. related, but apparently not all. She also tells Yamada that she will never fall in love with anyone else, and (assuming the translation is precise) it’s notable that she doesn’t say that she’s never been in love with anyone else.
I don’t know how significant any of it is, but I did notice that, except for the sports senpai, every time Kase has the opportunity to disavow any previous experience with girls (as often happens in yuri manga), she doesn’t do so explicitly.
And she certainly seems like a gal who knows her way around another girl’s blouse buttons and bra strap.