The shikigami of Riku Tachibana. Kogenta is a member of the Byakko (White Tiger) family of shikigami (leading many to the conclusion that Riku is Ten-Ryuu's Head). He is typically cocky, arrogant, and quick to anger. At first he can't believe that he's &quot;Stuck with someone as dimwitted&quot; as Riku, but eventually comes to terms with this and tries to improve Riku's character. He is the first Shikigami to become a Daikoujin, but he considers it an unfair advantage because, in his words, &quot;I can't remember how I beat them and by the time I come to, the fights already over.&quot; In the finale, Kogenta willingly lets Riku terminate their contract, but it is speculated that the contract still lingers, as Kogenta's spirit is seen over the shrine.

All his life, Riku Tachibana has been raised by his grandfather. For some reason, the old man has always been fond of strange hand gestures, and they've rubbed off on Riku, who performs them almost subconsciously, to his classmates' great amusement. One day, however, it suddenly becomes clear to Riku what his grandfather has been surreptitiously teaching him. And the teachings could mean the difference between life and death for Riku.
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(Source: Anime News Network)

The Onmyou Taisenki manga does not tell the same history as the anime. The story happens years before the events in the anime. The main character, Yakumo (who also appears on the anime) is the son of Monju, the chief priest of the Taihaku Shrine. In this shrine, Monju's former teacher, Mahoroba, is imprisoned. The problems start when Mahoroba escapes, gains power, and transforms Monju into stone.<br><br>
The story of the manga is about Yakumo's first steps with a Shikigami, training with Izuna (Taihaku Shrine's miko) and seeking Mahoroba through various Japanese historical dates. Famous Japanese historical characters such as Oda Nobunaga can be seen throughout the manga. (from the Onmyou Taisenki entry on Wikipedia with slight modifications)