I love the structure of 'Main Story + Aventure/Monster of the Week' of shows like Supernatural, Buffy, Smallville etc.

But is really hard to find korean shows like this for me.

So any recommendation? Thanks in advance.

 Also Tomorrow but that's very heavy and deals with very sensitive topics, just as a warning.

Thank you very much emeraldarrows. I'll add the four shows in my watchlist.

I second Tomorrow, it's great (but again, tw)

Also recommend Taxi Driver and The Master's sun. Hotel del luna also fits your request, but I thought it was mid.

Revenant has one big baddie, but it also features different ghosts in each episode--at least early on.

And I don't know how open you are to older dramas, but Secret Investigation Record is a good one. It's arguably closer to The X-Files than Bufffy or Supernatural, though. If you don't mind the lower production value--especially compared to new dramas--I really recommend it.

probably Vampire Prosecutors since it has vampire as a prosecutor and the enemies are also vampires

And these may not similar but:

A Korean Odyssey (human and monsters dealing with monsters)

Tale of Nine Tailed 1938 (monsters dealing with monsters)

The Uncanny Counter (human who's supposed to be dead dealing with ghosts)

Moving (people with supernatural powers dealing with people with supernatural powers)

But I actually haven't watched Supernatural so I may be wrong