Tsubaru is an acquaintance of An Lili from a town called Tousui. They met when Lili was trying to get Nadai banned from the town. Tsubaru runs an inn in Tousui.
Yona along with Hak, Yoon and the Dragon Warriors are introduced to her when they begin to investigate nadai being brought in from the Sei Kingdom. They also learn that people are going missing in town and that Tsubaru's son was one of the victims.
Tsubaru serves Lili a laced drink which causes her to pass out and she covers Yona's mouth and forces her to inhale a knockout drug. She sends them to Sei to work as slaves. When the girls wake up in Sei, Tsubaru reveals that she is a nadai merchant from Sei and does not have a missing son. She has been serving nadai as "medicine" for the slaves to keep them working and compliant, despite serious injuries.