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In manga chapter 101 Riyo is shown to have a similar slave tattoo to Enjin. Enjin stated one of his goals was to help kids sold into slavery. Riyo’s also shown to have issues with her parents. My theory is Riyo loved her Mom and Dad but her love got twisted into her killing someone. Maybe her father? And she was sold away afterward. Her person she was sold too used her as a tool for assassination put her through torturous training and had her kill a lot of people. Ends up crossing paths with Enjin (maybe by contract) and gets saved, tattoo removed or hidden, and told to join him start over and to never kill again. (Cooking or delusional 🤔)