The nephew of Toyotomi Hideyoshi's wife Nene. Inheriting the Kobayakawa family as the adopted son of Kobayakawa Takakage, he became a great daimyo with 570,000 koku under the Toyotomi regime. He positioned himself on Mt. Matsuo to watch developments until the very day of the Battle of Sekigahara (1600), and finally defected to the Eastern Army after Ieyasu had gunfire signals sent toward him. This betrayal decisively determined the outcome. After the battle he was treated favorably as lord of Okayama, but became mentally unstable and died in 1602 at age twenty. The judgment that "the outcome at Sekigahara was decided by Hideaki's betrayal" remains unchanged today.

Young and indecisive, unable to make a decision between two great powers. Often depicted as history's betrayer, but it is also possible to see the tragedy of a young man of barely twenty overwhelmed by the pressure of his era.

Inscribed in history as the person who determined the outcome of the Battle of Sekigahara. Along with the lesson that "indecision can change history," his act is told as one of the most famous "betrayals" in Japanese history.