There was a reason of why Yufa became evil that was because she used to be a childhood friend of Roan as he made his vow to protect her from evil. However when Roan returned to the party as a crusader, he eventually became ignorant and arrogant as he completely ignored Yufa most of the time and even blamed her during the dinner in Alberta at one time. The whole situation caused Yufa hurt and upset of his behavior.
Turning to evil
During the attack in Comodo, Yufa was approached by Keough as both of them began to exchange glares with each other. Eventually, Keough tricked Yufa into going near him and it was a success. When Roan went to look for Yufa, he was shocked to see Yufa hugging Keough. After Yufa bid farewell to Roan, both the siblings immediately departed for Glast Heim as Roan could do nothing but watch on in despair.
Once in Glast Heim, Yufa was given a meal. While she was eating her meal, Keough walked into the dining room and thanked Yufa for a sacrifice and told her that "the new world" will be born. Upon hearing this, Yufa became excited and she told Keough that she was trying to help him. When Keough darkly asked her of why she left Roan behind, Yufa became confused and surprised. Then, Keough walked towards Yufa and tell her that she was desiring to be loved but not before he also reminded Yufa that as long as she was siding with him, she will not worried anymore. Yufa became happy as she asked Keough of what she was going to do. When Keough told Yufa to stay in Glast Heim with him, she became very shocked. But before she could say more, Keough suddenly thrusted his hand into Yufa's body as she looked at her body in despair. Keough then took the soul crystal out from her body and held it in his hand as he began to laugh. Then, Yufa says: "Nii-san, i am... iam..." just before she fell on the floor. As Keough was laughing maniacally, Yufa's eyes began to turned red as she became Dark Lord's new puppet.
The final battle and returning to good
Yufa was then ordered by Dark Lord to sent the army of the undead against her friends. After a tough and intense fight, Yufa appeared before Roan and the others and proceed to teleport them to Glast Heim for the final battle.
When the rest of the party were battling with their enemies, Roan went and looked for Yufa. When Roan finally located Yufa, he however could not hear her words. Yufa then hugged him and began to kiss him. Yufa then slipped the cursed ruby into Roan's mouth as Roan began to struggle in pain as he slammed on the floor in pain as Yufa watched on in delight.
Taunting Roan of his objectives, Yufa then proceeds to impaled Roan on his chest with a cursed blade.