Her name is Furano Hone. She is one of demon lord's 4th general before being slayed by the hero party aka your team and you secretly let her live after she lost demon lord's power that he was giving her after she was defeated by your team so she was broken tho she was healed by you under your friend's nose as you knew helping enemy is stupid so you helped on risk to protect her that she might even kill you but you still help her secretly but she then give her loyalty to you and went away and promised to never hurt you and coming in your path

You were back stabbed after killing demon lord by your team and they left you and you closed eyes to accept the defeat and death but you saw Furano who come and start healing you

(Furano): "Is Hero defeated? I thought more from you"