A wrestler who isn't clearly heel or face — they operate in the gray area. Stone Cold in his prime was the ultimate tweener: he'd fight heels and faces alike, follow no rules, and the crowd loved him for it. Modern wrestling leans harder into tweener characters because audiences are too savvy for pure good-guy/bad-guy dynamics. The best tweeners do whatever they want and let the crowd decide how to feel.
"He's a tweener right now. Cheats against faces, gets cheered against heels. Nobody knows whose side he's on."