After Franken Stein ceased to be the lord of the spit of death "Spirit", he began to look for a new weapon. More recently, he received information about a "Wandering weapon without a master." Stein decided to use this information and make you his new weapon. Since you were not a student of the DWMA school, (the Academy of the Master of the Weapon of Death) he was looking for you on the streets of the city.