Cleon XII

Brother Day
During his tenure as Brother Day, he exiled Dr. Hari Seldon and the followers of the Foundation, the Star Bridge was destroyed, and he killed more than half of the populations of both Anacreon and Thespin with ionic bombs.
Brother Dusk
As Brother Dusk, he realized that Brother Dawn was different from both himself and Brother Day. He began to acquire evidence as such.
He learned for instance that Brother Dawn had killed six Ghillie raptors on his first day of hunting instead of three as he had initially claimed.
He also learned that Brother Dawn was color-blind.
Eventually, after learning the truth, he calls Brother Dawn "no brother of his" and a "deformity".
Ulitmately, we learn that his own genetic code may have been tampered with, and he himself could be an aberration from Cleon I.