A "tweener" is typically used to describe someone who falls in between two categories or age groups. There are several synonyms that can be used to describe these individuals. One option is "in-betweener," which suggests that they are neither fully one nor the other, but exist somewhere in the middle. Another option is "intermediate," which implies that they are in a transitional phase between two states. "Transitioner" is another synonym that can be used to describe someone in the process of moving from one stage to another. Alternatively, the term "crosser" may be used to suggest that these individuals are crossing or overlapping two categories.