Isako was the leader of the Katakuri Ninja Clan that operated out of Inotakimura, Japan. She and her clan serves as killers for the Director and H.A.R.M., primarily out of duty than a sense of allegiance. The Director has stated to dinner guests that he must manipulate Isako much like a little dog (much to the anger of some of the clan members) but also recognizes her honor and competence in his happy birthday note he wrote to her (as later found by Cate Archer).
In her earlier years, Isako was alone on the streets of Kyoto (after her parents had been murdered) when she was found by an old woman and her daughter (whom at the time was the head of the Sakuragaoka Ninja Clan) where Isako was trained in the ninja arts. When Isako turned 14 years of age, she was sent on her first mission. She was tasked with killing a merchant who was kidnapping girls to turn them into prostitutes for his gambling dens. However the mission was a trap set up by the man who hired the ninjas for the task and Isako was captured before she could complete the assassination. Before the merchant could dispose of Isako, his European friend (The Director) asked if Isako's life could be spared so that she may work for him. Thus the deeply honorable Isako felt she owed a great debt to The Director. She turned down several offers to betray The Director out of this sense of duty.
Isako was given the task to kill Archer in Japan, but Archer survived the stab wound. When it was discovered that Archer had survived, Isako and her ninja clan was sent again to complete the task tracking her down to Akron, Ohio. After Isako had failed again, she has fallen from grace. When The Director tried to shoot Isako for her failure, Cate Archer saved her life by disarming the villain. Thus freeing Isako from her bonds of honor to H.A.R.M.