why does minami like yoshii?

doesnt he keep saying shes flat and all that? not to mention not having any qualities at all eg. intelligence/looks/wealth/personality/humor (other than being picked on)

Being flat may be a compliment to / from some

What exactly he thinks of Minami isn't entirely clear, but it's quite clear that the one he's most interested in is Himeji. He's been calling Minami unfeminine (and seriously implies unattractive) from the get-go (with flat being just one of the complaints on the list), so while he may care for her as a friend, I really don't think that he has any interest in her romantically. Where it will go, however, we'll just have to wait and see.

it all comes to one simple concept actually....
Tsundere