Tweener

between + -er, i.e. someone who is caught in between

Full definition of tweener

(plural tweeners)

(sports) A person who plays at two positions, not good enough at either to be restricted to it.
(tennis) A shot that is hit between the legs. It was first popularized by Frenchman Yannick Noah.
(computing) A computing device that is smaller than a traditional laptop, but larger than a PDA
(professional wrestling slang) A professional wrestler who is neutral or morally ambiguous; neither a face nor a heel, or showing no favoritism to either allegiance.
(cinematography) A person, machine or piece of software that performs tweening
(cinema) A film that falls between two genres (and thus may not be easily marketed).