Eoltteolgyeore Namju Dongsaengeul Guhae Beoryeotda

Charlotte Rania thought she'd be free to enjoy her life as an unimportant side character... until she saves a dying boy, who turns out to be Theodore Heinst, the younger brother of the male lead! Despite her reluctance to get involved with said male lead, Duke Carlyle Heinst, Charlotte has no choice but to help him when she discovers that mysterious forces are trying to kill Theodore. It doesn't take long for her to realize that changing a character's fate comes at a price...

(Source: Tapas)