ML and FL love each other a lot and really want to be together but fate/things/people are actively trying to stop them from being together. Can be a tragedy (but dont give me spoiler. Dont tell me if its a tragedy or not). No matter what they do, things are not in their favor.
Example:
Hotel Del Luna
Gyeongseong Creature
Angel's Last Mission Love
Till the End of the Moon
Love and Redemption
Moon Lovers
Goblin
Crash Landing On You
You Who Came From the Stars