It's a very different kind of story from what's on this list and i dunno why BUT I JUST REALLY LOVE THIS STORY OK
Hands down the best webtoon I've ever read. The art is fantastic, the characters have growth, the premise is excellent, the plot is engaging... I wanted a slightly more robust ending but everything is honestly so fantastic that that feels like a nitpick.
Don't let the cover fool you. It starts off as a bit of a comedic vampire setting, turns S&M very quick, and then has an excellent plot that picks up about half way through. Set aside a few hours to read it in one sitting!
You've finished read
My poor heart
Kimi wa Natsu no Naka