Emperor Uriel Septim

The 85ish year-old emperor of Tamriel. He has been the protagonist of all 4 ES episodes, but in the 4th chapter he will perish in the start of the game, by hands of the unknown assassins who will raid the palace. In his dying breath, he will ask you to find his long-lost son and will hand you the Amulet of the Kings, the heirloom of the royal crown.

The Heir

No one knows who he is or from where he comes from. Some claim that the Heir is in fact Callaxes, who was exiled during Jagar Tharns rule, but alas, he is the last living son of Uriel Septim and the only hope that Tamriel has to escape the nearing end of the Daedric invasion. He will be a valuable ally in the final moment of the Main Quest.

The Dark Priest

The leader of the Mythic Dawn Cult and final boss of the storyline. He is responsible for the emperors death and the Daedric invasion. It was him who had slain the emperor bellow the palace dungeons and escaped the guard unharmed on the orders of his master, the Daedric god Mehrunes Dagon. Little is know about the Dark Priest, except that he comes from a line of distant kings (possibly ayleidic).

Dremora Kynval

One of Mehrunes Dagons strongest lieutenants, he lead the assault on Kvacth and is responsible for the towns destruction. Arrogant and ignorant like most Dremora nobles, he looks at mortal life with disgust and hatred. He is amongst the first Daedra to die.

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TES 4: Oblivion Characters - uber Oblivion Portal edition
Here for old time's sake.

Fargoth. After being driven to complete madness by his ring, or more accurately the lack of him ever having his ring, he ran far from Seyda Neen and found his way into the care of a daedric prince. After months of resting his strength grew to untold highs, his quest for revenge giving him a greater purpose, one by one the daedric princes fell before him, leaving him as the unchallenged ruler of all of Oblivion. He returns as Lord Fargoth to complete his revenge on the inhabitants of Tamriel.

The Creeperescapes in what some might call a daring escape - with all of his money, leaving countless untold thousands of player characters struggling through the game having to deal with the cheapskates in the rest of Vvardenfell. Nobody yet understands "Dagoth" Creepers' real purpose - to bring great value to all the people of Tamriel - and beyond.

Upon the edges of Oblivion before the knees of the Dark Lord Fargoth, Dagoth Vader announced that he was Fargoth's father, Lord Fargoth fell in tears before his daedric armies, Vader taking his hand and pulling him onto his feet - announced they would rule the galaxy as father and son.