Eichiro was the son of Eiji Tanaka, a fellow general of Benju Matsumoto. During a battle, Eiji betrayed his army but Benju managed to win and executed Eiji for treason. Eichiro however believed that Benju killed his father out of jealousy over the man's superior generalship and promised vengeance.
In 1358 DR, he hired the ninja Yamaguchi clan to kill the pilgrims going to the local Shining Temple of Bishamon. His goal was to ruin the Ceremony of the Three Thousand Steps and become the next daimyo of Aru, after Benju's demotion.