Having sported a personality to match his equally unappealing features, Eiken was a perverted drunk with anger issues. Although he might have appeared to be dimwitted, it has been debated whether his plan to overthrow Tasuki was smart and bold or just a stupid drunk fantasy to begin with. His actions, while fueled by either rage, lust, or envy, could have been attributed to his constant inebriation and seemingly the lack of basic education. Much like a child, he was prone to throw temper tantrums, whine when he did not get his way, and crumble in the face of defeat.
He was slightly prominent in Genro Den, Tasuki's light novel,
After Tasuki left to find a cure for Hakurou's illness, Eiken lurked in the shadows and waited until his leader passed. When he did, Eiken took up command of the bandits and stole the blessed Tessen, a diamond fan that had the ability to conjure up flames when waved. With this weapon, Eiken turned the bandits against Tasuki, who was rightfully their leader by Hakurou's orders, and he was sent into hiding, only revealing his true location to Kouji, who reluctantly followed Eiken's orders.
When Nuriko, Hotohori, and Miaka were captured during their travels, Eiken ordered his men to take Miaka, who he was attracted to, into his room. Meanwhile, the other two warriors were mistaken for women and were forced to serve the bandits drinks. Luckily, the perverted false leader was interrupted by Nuriko and Hotohori before they were all ambushed by a cloaked figure, soon revealed as Tasuki, who took Miaka, thinking he could hold her for ransom. In a rage, Eiken attacked, which allowed Nuriko and Hotohori to escape and look for their Priestess.
Tasuki and the rest of the party return and battle Eiken and the loyalist bandits. In the end, Tasuki took back what was rightfully his and defeated Eiken.
After his exile from the Mt. Reikaku bandits, Eiken retreated to join their rival: the Mt. Kaou bandits. It was there he gathered an impressive force to attack his former gang, kill Tasuki, and claim dominance over the two mountains.