Sanshi is part of the Xuanwu Turtle clan, which practices an ancient ritual when they are 15 or so, to take the virginity of a young girl in order to completely awaken their bloodline. In this case, it was Nannan, who volunteered herself because her mom was sick and the clan elders promised to give her money and medicine to treat her mom.
Due to Sanshi's inexperience, Nannan ends up being traumatized by the event, and afterwards she flees to Shrek Academy with a deep hatred of Sanshi and scarred heart. Once Sanshi found out why Nannan volunteered, and that she was bribed by his family, he cut ties with them them and he followed Nannan to Shrek Academy in order to take responsibility for his actions and because he was genuinely in love with her
During the Sea God's Dating Festival, Sanshi was finally able to date Nannan, however she was still traumatized by the previous events and their relationship progressed very slowly.
During the Yin-Yang Love Querying Valley's Trials, Nannan discovered that while she was at Shrek Academy, Sanshi had been visiting her mom almost daily and taking care of her, making sure that she did not get worse, while Nannan had not seen her mom even once since that night. Additionally, Both Sanshi and Nannan got to relive that night, and when Sanshi decided not to take her virginity and give up his chance as the successor to his clan out of his love for her, she was finally able to let go of the shame and hatred she felt for him. They were then able to complete the ritual correctly and become a happy couple.