Going through most of the Challenge seasons and punching the confessional counts into a normal distribution calculator. The ones in the highest tier I'd consider to be the "main characters" of the season.
All Stars 2: Nehemiah Clark
Spies, Lies, & Allies: Tori Deal & Devin Walker-Molaghan
All Stars: Jemmye Carroll, Mark Long, Darrell Taylor, & Aneesa Ferreira
People Who Were Main Characters Multiple Times: Tori Deal (2), Devin Walker-Molaghan (2), Kam Williams (2), Paula Meronek (2), Brad Fiorenza (2), Susie Meister (2), CT Tamburello (3), Laurel Stucky (3), Mike Mizanin (3), Wes Bergmann (4), Cara Maria Sorbello (4), Evan Starkman (4), Johnny Bananas (8)