Manthika

Some dramatic irony.  Reincarnated as the saintess-to-be in a novel she knew well- the main character managed to accidentally get attached to the demon king.  As the saintess.  Without realizing who he was.  Amazing.  In her defense... they met on a blind date while he was in disguise and he kept his identity from her.   But they're just friends at the start!  She absolutely isn't instantly head over heels for him.  Lying summary... she's just trying to live her life. 

It's a bit confusing at the start, because they hint at everything instead of saying things flat-out, but it's entertaining enough.  Even if they're awkward as heck.  

Fake/Pretend relationship, unrequited love interests, romance, drama.  The ex-boyfriend hardly plays a role, so talk about misleading titles, lol.  The summary isn't much good either.  But the manhwa itself, yeah, it's alright.