Tweener is a wrestling term for a morally ambiguous wrestler or e-wrestler, who is neither a bad guy (heel or (good guy (face). He (or she) who will fight anyone regardless of alignment. This term is also used to describe wrestlers who use tactics typically associated with heels (such as cheating), yet are still cheered by fans in spite of (or because of) these antics. It can also describe a character who strives to be a fan favorite, yet due to circumstances beyond his control (such as a psychological disorder), he unwillingly resorts to heelish tactics.
In Full Metal Wrestling, the tweener alignment isn't recognized due to the constraints it puts on the ability for the staff to book storylines.