Eichiro was the son of Eiji Tanaka, a fellow general of Benju Matsumoto. During a battle Eiji betrayed his army but Benjo managed to won 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 in order to kill the pilgrims going to the local Shining Temple of Bishamon. His goal was ruin the Ceremony of the Three Thousand Steps and become the next daimyo of Aru, after Benju's demotion.