An inbetweener, as implied by the name is a male neither a boy nor a man,it is that perfect inbetween stage where looks are still pumping strong but nothing is yet wrinkled.
Also this stage allows for the awesomeness of a younger male with the maturity of a man.
An inbetweener is not fickle and wants to have fun.

Girls love the inbetweeners because they are the perfect combination.