So yes this series was adapted from a webtoon and yes the webtoon itself was quite popular. But just because the manhua was good doesn't mean you have to make into a series. Webtoon have much more episodes and are in general pretty detailed. A lot of adaptation fail to meet the expectations (like cheese in the trap, mystic pop up bar), some are average (like extraordinary you, love alarm) and a few turn out pretty great (like what's wrong with secretary kim or my ID is gangnam beauty). The failure of this series is evident in the fact that despite having stars like Yeo Jin Goo and Lee Jong Hyun in the main roles, most people disliked the series.
I started this series because the vampire concept is mostly only adapted in hollywood mainly due to 2 facts-
-most countries have mythical creatures different than vampires in their folklore (the reason everyone knows about vampires and werewolves is because they are more publicized) & hollywood's has romanticized them even though in reality they are more related to horror and less to romance
So this was definitely something interesting especially since in the show the humans are said to be aware of the vampires and living together in an integrated society.
This interest lasted about 2 episodes. You would think that a show with vampires cohabiting with humans would concentrate more on their struggles but no, the writers had to add in-
-a complicated teen romance,
-b*tchy highschool girls hellbent on ruining the new girl because the queen bee's crush seems to like her ( if the guy didn't give you the time of day even before the new girl came to school, you think he'll do so after you bully her),
-the vampire girl joining the school band because she wants to be a musician (seriously that's her plan? dude just become a trainee in an agency or join an art school or something that'll actually help),etc.

The worst part of this drama was probably when they switched time periods. Look I like period dramas as much as the next person but there is a time and place for that. I guess the writers wanted to show a reincarnation story or maybe a "history repeating itself" kinda plot but it seriously failed. There were way too many things happening and the structuring of the episodes was a mess. Not to mention that Ma-ri's character was way too dull and Jae-min's character was uninteresting (Shi-hoo was a more intersting character).
If you wanna watch a korean vampire drama I suggest you go for Blood or the Scholar who walks the night.