I didn't really get the cover image. “I wonder what I really think of her”? I assume it's Yuri, but they're already past the “I wonder if we're friends now” hurdle. I mean, I guess everyone's yuri jokes could actually be real and Yuri's at the point of “I wonder if I'm romantically in love with her now” (in which case, a lesbian character named Yuri? Oh you, Tanigawa Nico…), but while the series obviously flirts with the idea, I haven't gotten the sense that it has or ever will get beyond jokes. (Although I did find interesting the read on Tomoko as a gay girl who hasn't internalized she's actually gay; her “sexual harassment” would stem from that coupled with her social ineptitude, and would be seen as an inappropriate way of showing affection.)
On my first read-through, I in my limited understanding took the original Japanese “私の中の気になるあいつ” as “[this is a] person interested in my [= Tomoko's] inner [life]” i.e. “interested in getting to know me”, which implies someone who isn't already a friend. So honestly, I thought it was someone else. (I was unsure who, though; layout-wise I was reminded of the panel with Wada with whom Tomoko discussed manga in chapter 144 page 7, but that'd just be out of left field.)
In the end, I guess it's Yuri and it's either a yuri thing or, (in my opinion) more likely, Yuri wanting to become best friends with Tomoko.