Each Once is given 9 lives in this dimension. And a guardian must accompany them all throughout all those lives. A Once is a person who needs to be guarded from the darkness of the Thrice dimension, and Twice are the guardians.

Yes, being a guardian was easy, but it takes more than a heart of stone to be one. Being it has one rule, "Never fall in love with your Once."

Nayeon was the first guardian to break that rule as she fell for Jieun. She was her first once, and it was lucky for Nayeon that it was Jieun's first life. Jeongyeon became Jieun's 2nd life Twice, but she ended up in the same hole Nayeon fell in.

Momo, Sana, Jihyo, Dahyun, Chaeyoung and Tzuyu also became Jieun's guardians. All of them ended up in the same hole as Nayeon, and all of them were placed into the 4Dimension as a punishment from the higher ups, which is the JYPNation, for breaking the one rule that all Twice's had. After much deliberation, the higher ups have no choice but to stop all these rule breaking, And here comes Mina.

Myoui Mina. The one and only Twice guardian who came from the Thrice dimension herself because she herself fell for her old Once, and because of theat she was cold and quiet. She learned how to not get attached and the JYPNation gave her her privilege to be a Twice guardian again. She guarded a lot of Onces after getting being a guardian again without showing attachment to them so the JYPNation sent her to guard Jieun on her final life. The higher ups trusted Mina to break the curse. And if Mina didn't succeed, her punishment is to be sent back to the Thrice dimension.