A long time ago, Yakage was in the service of Washu when she combined her own DNA with one or more Mass to create her daughter, Ryoko Hakubi. Yakage also saw potential in clones created from Mass, so he took a partial DNA sample from Ryoko, completed it with copies of his own X chromosomes, and combined the resulting sample with Mass to create Minagi, who he adopted as his own daughter.
Yakage had a dream to research as many sciences and swordcrafting arts as possible to produce a blade called the "Lightning Eagle Sword", which he intended to give to Minagi once complete. He acquired Kageba and intended to study it to advance his research, but Hishima of the Shima Brothers appeared and threatened to kill Minagi (who had been placed in a healing tank to recover after one of their sparring sessions) unless he surrendered Kageba to him. Yakage had no choice but to allow Hishima to leave with the blade so that his daughter could be safe. As Yakage lifted Kageba in defense of someone he cared for, it generated a powerful aura around him, but when Yakage handed it over to Hishima, the aura moved to him, as if Hishima was also trying to protect someone he cared about by stealing Kageba. Yakage later recorded this experience in his own personal journal.
Years after Yakage's death, Minagi discovered his journal and read about the theft of Kageba from his perspective. Minagi began to blame herself for the theft, so one month later, in 150,081 CMT, she tracked down her mother and sister aboard the Bebop II and told them that she wanted to find and reclaim Kageba. Minagi's striking resemblance to Ryoko confused and upset Tamao, so Kokonoe told her about Yakage and how he created Minagi. Minagi began to tell the group about Kageba but broke down into tears, so Washu comforted her, saying that Yakage was always proud of her and that he would not want her to be upset, especially when she had new friends to make.