Yusuke and Botan
I am by no means a Yusuke+Botan fan. And I don't know of anyone who is. I am totally Yusuke+Keiko. However, have you ever noticed the series has these moments that just shout "Forget Keiko it's you I love Botan!" (that'd be Yusuke shouting of course). I'll give you some examples below.
In the second movie (the one where Botan in unconscious, like, the whole time) when Botan first appears Yusuke runs to her in tears and pushes everyone else out of the way so he can catch her. Every time the villain, Yakumo, says something about her or tries to touch her Yusuke goes all crazy and does this weird glare that he only does when Keiko is in danger.
In the last episode of the first season when Yusuke jumps out of the way of Toguro's attack he puts his arm around Botan's waist and takes her as well, but then he doesn't let go.
There are more too, and I'll find them when I start the episode summaries.
Honestly, I think it's just because she's a girl. Or maybe because she's the one who helped him and still does. I don't think he treats Shizeru and Yukina like that. I just find it interesting.