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.

Riku Tachibana is an average 1st year junior high school student who was supposed to be living a peaceful, ordinary life, but just after entering junior high he ends up fighting a duel against a mysterious man calling himself “a Warrior of Earth,” and Riku’s life is changed forever! Riku discovers that he is a survivor of the believed extinct “Warriors of Heaven,” Using a device called “DRIVE” and summoning a Shikigami, or “Divine Beast,” called Kogenta to his side, Riku devotes his life to the battle against the Warriors of Earth!
 
(Source: Sunrise)

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, is the son of Monju, the chief priest of the Taihaku Shrine. In this shrine, Monju's former teacher, Mahoroba, is imprisoned. Problems arise when Mahoroba escapes, gains power, and transforms Monju into stone. 
The story features Yakumo's first steps with a Shikigami, training with Izuna (the Taihaku Shrine's resident miko,) and seeking Mahoroba through various Japanese historical dates. Famous Japanese historical characters such as Oda Nobunaga can be seen throughout the manga. 

 Source: Wikipedia, Edited.