Hovering between the "popular group" and other sorta popular groups but not a full member of the group. Could be kicked out of the popular group. Or hang out with the popular groups not in their grades. Socially hated or accepted by most groups.

someone who is sexy beyond mean

Being semi-popular would mean you are either apart of many friend groups (including the popular kids group- or maybe just a couple of groups) or you are well known around the school where every knows your name but not many people talk to you

They are usually seen as the misfit (or quiet kid) of the popular kids group but the cool/chill kid of a ‘lower class’ friend group.