Ryuu's the oldest of Koyama Akemi and Toshio's three daughters. Her mother was a shinobi.
Japan remains torn asunder by civil war in Ryuu's native reality. Her insistence on using hand-to-hand combat instead of ranged combat in battle showcases her strength. She abstains from using weapons with the potential to reach more than three shaku from her hand. On top of that, her choice of weaponry varies according to her mission. She wields a pair of sai for assassinations whereas she carries two reverse-blade wakizashi for capture missions.
However, she didn't decide to become a shinobi until she and her father returned from visiting the smith Tajiri to find her mother and sisters dead. A few days later, the Koyamas returned to Tajiri's forge and commissioned Ryuu's first pair of sai from her. Thus, they resumed their family's patronage of the smith's family.
About three years later, Ryuu received the scars I mentioned earlier during a mission. However, she takes the greatest pride in them because she escaped with what became her first reverse-blade scythe. No more than a month later, another visit to Tajiri's forge resulted in the commission of her first reverse-blade wakizashi.