Yeonji got got into an accident with her lover Jaeyoung...she lost her eyesight from the accident while he died...but she never know about the fact that she had loose him as someone literally had act as him

Jungkook who had been secretly loving her disguise himself as Jaeyoung...He had never approach her before since he want to keep his feeling as secret...He want to keep his distance from her as he know she is out of his league..she is his twin brother lover...(yup he is Jaeyoung twin)...but after he learn about his twin death and Yeonji's condition, he decide to act as Jaeyoung in front of Yeonji..He knows how deep Yeonji's love is to Jaeyoung..that is why he decide to do that, he doesn't want Yeonji to blame herself for everything that had happen...

He must have not look the same as his twin brother ( since they are fraternal twin ) but his voice does..they sound exactly like Jaeyoung's..He use Yeonji's condition as an advantage for him to disguise himself as Jaeyoung..