In eps. 10-11, Su Su and Bai Yue Chu are stuck in one of the strings that the yao guai pulls. To escape, Bai Yue Chu writes a spell on Su Su’s forehead. (She thinks it’s a kiss, lol.) The spell activates 纯质阳炎, a special fire that can rid of yao guai’s powers.
Bai Yue Chu being Bai Yue Chu: Asks for a year’s worth of candy in return for saving her. Su Su readily agrees.
Su Su’s older sister (Jia Jia?) muses “This fire seems to be able to change the destinies of these two. How to say it? This type of piercing yet empty feeling, after so many years, is it still the same?”
Su Su’s briefly transforms into Hong Hong, triggered by Bai Yue Chu’s blood (he was swung pretty hard in the yao guai’s strings) and the fire. She’s back to Su Su before Bai Yue Chu can look at Hong Hong though.