Is this The Villain's Savior...?
Set on a path to tragedy and misfortune from a young age, Aseph Randell is doomed to die a villain. That is, until the mysterious Elzay Tiathe appears in his life with a promise: "I can save you." After having vivid visions of him for so long, can Elzay untangle the twisted fate tied to Aseph...or will they both be dragged down together?
A woman reads a novel titled "The Crest of God," set in a world in which matching pairs of crests magically appear on people's bodies to indicate who their soulmate is.
The woman becomes particularly attached to the villain of the novel, Azef Randall, who endured abuse and misfortune from a young age, fell in love with the heroine of the novel, Elisa Thyase (whose crest he did not share), and ultimately died heartbroken at the blade of her soulmate, Alchester.
So attached does this woman become to Azef that she has dreams about him while reading the novel, and cries herself to sleep after reading of his death, yearning for the ability to save him.
She then wakes up within the world of the novel as the twin sister of Elisa, Elljae Thyase -- a character who didn't exist in the original novel -- and sets about trying to change Azef's fate. Based on what I've read, the story appears to be a romance between the two of them going forward.