Brother Day

The Day on the throne when the story opens puts Hari Seldon on trial for predicting the Empire's fall. After the attack that brings down the Star Bridge, he agrees to send Hari and his followers to Terminus, far from Trantor. By the dynasty's tradition, at each cycle Dusk is disintegrated in the Ascension, Day becomes the new Dusk and Dawn takes the throne. Demerzel, always at their side, makes sure the cycle never stops.

Decades later, a new Day faces Zephyr Halima, a religious leader who questions whether a clone has a soul, and takes on a journey no Cleon had ever tried: the Maiden's Spiral. If he completes the pilgrimage, he proves a clone has a soul too and calms the religious unrest. Meanwhile, Brother Dawn's romance with Azura turns out to be part of a plot to bring down the dynasty. Day takes revenge on Azura's entire family, Demerzel kills Dawn, and Day learns he may have been altered himself.

In S2, Day survives an assassination attempt and negotiates a marriage with Queen Sareth. He has no idea Demerzel played a part in that attack. At the same time, Dusk and Enjoiner Rue grow suspicious of Demerzel. Day brings in General Bel Riose to investigate the Foundation, goes to Terminus in person to confront Seldon and orders the Invictus brought down on the planet. In the end he falls into Hober Mallow's trap, swaps places with Riose and suffocates in space.

152 years later, another Day reigns with his consort, Song. The Empire has lost its jump drives, and the Mule is rising on the fringe. Young Dawn takes an interest in psychohistory and ends up dead. Day removes the nanites that heal his body and vanishes into Mycogen, where he is tried and sentenced to death. When Song goes missing, he goes looking for her. Back on Trantor, he makes peace with Demerzel and shows her an ancient robot head. But Brother Dusk refuses to die as tradition demands. The two fight, and without his nanites, Day can't heal from the gunshot and dies on the palace floor.

Brothers Day, Dusk and Dawn don't exist in Isaac Asimov's books: they were created for the show. Cleon I, the emperor the clones come from, appears in Prelude to Foundation and Forward the Foundation.