@bastek66 i LIKED the premise. The problem isn't that she is supposed to be an annoying big-titted kouhai, it's that after 50 chapters she is ONLY that.
If you want to write a main character with an obvious big flaw in personality you also need something to balance it out or after a while it simply become unsympathetic (and no, fanservice doesn't work for that).
For some reason the author went the exact opposite way, putting even more focus on her flaws, which now, in the eyes of many readers, overshadow her good points.
If you focus all the attention on a char being annoying/narcissistic, don't be surprised when that character is perceived annoying/narcissistic and as such, DISLIKED.