Tomorrow
While Rowoon plays the main character, it's everyone else that you want to focus on. The backstories are absolutely incredible, and there's just enough whimsy in there to not make you super depressed, despite the inherently dark plot (they're grim reapers who stop people from ending their own lives instead of bringing them to the afterlife). It was only the second Kdrama I'd ever watched, and while it's been well over a year since then, it still holds its place as my current favorite asian drama ever.
Weak Hero (Class 1 & Class 2)
A true MASTERPIECE. You will never see Park Jihoon the same after his role as Sieun. Since it's short (16 episodes over 2 seasons) I tell people they literally have no excuse not to watch it. While some of the actors seem too old to play their characters, trust me in that you will forget about it once this show rips your heart out. Note: watch this alone or with only friends you trust enough to see you ugly cry.
20th Century Girl
This is a movie, but it's too good to not recommend. Another one to watch if you're prepared to cry. I watched this with my best friend around the time it came out in 2022, back before I ever watched Kdramas or was even into Kpop. And my God, it is beautiful. Definitely a must-watch for any romance lovers.