Sungjun (Park Seojoon) in She was pretty.

I'm not sure if Doguk (Sung Hoon) in Perfect marriage revenge falls into this category, because I just remember he has water trauma, but I'm not sure if it involves rain.

Business Proposal and Wedding Impossible are the only two they came to mind straight away. Maybe there are others I have forgotten like She was pretty, thanks @claire2009 for the reminder.

He'll come as the rain. He'll come as the first snow.

Sometimes the ML causes the rain trauma! Eun-tak's chant of Kim Shin's words. Goblin, episode 14.

If I remember correctly, the ML in Seasons of Blossom (Seo Ji Hoon) didn't like rainy days (he says he doesn't like when Summer comes because of the rain).
But it's not such a big trauma as in Business Proposal or Wedding Impossible.

I believe Dear M has a ML like that... or is it the FL? Now I'm questioning it, lol

I with kdramas would understand traumas dont need triggers to manifest. the brain itself is the trigger, it doesnt depend on outside circumstances / conditions. If you get a panic attack there is often no trigger, no warning, it is completely out of the blue and disconnected from the situation. thats why it is even scarier.
so there cannot actually be such a thing as a trauma + thing. bc if you gonna have a panic attack you gonna have it whether it rains or not. Trauma doesnt wait for the "right conditions"

that said, if it was only rain all people who actually experienced panic attacks would rejoice, to be able to predict and take countermeasures. but you cannot! thats the thing

While not trauma inducing our lovely angel from Angel's Last Mission needed to keep out of the rain due to his wings popping out when wet.

Well prior to meeting Sol uri Sun-Jae had rain trauma in Carry Sun-Jae and Run (Lovely Runner).

Thereafter now only he started liking rain but wrote a super hit song too!!

PS - Sudden Shower currently has 13M views on YT and is in the Billboard Global 200 chart.

I had just written about Wedding Impossible ML and his rain trauma a few hours ago on my review and then saw this article lol.

I have a female lead -- Park So-dam from Record of Youth. When she was a child, her mom had left her at school, and she had to watch the other kids get picked up by their moms with umbrellas, while she had to walk home alone in the rain, so she associates rain with loneliness.

since I cannot think of any more I still propose (again): a silhouette on the moon a la E.T. - one example being My love From The Star drunk alien Joseon era mishaps.
another good treasure hunt would be makeover gone wrong with horrible haircuts

Secretary Kim - Egotistical chaebol is triggered by red shoes, which dredge up the buried memory from childhood, when he was kidnapped by a scary lady in red shoes who committed suicide right in front of him.

Oh My Venus - Chaebol/trainer's secret trauma has something to do with a terrible leg injury he suffered as a teen.

Kill Me, Heal Me - I think this one's my favorite. The ML's childhood trauma, involving being kept locked in the basement and also a fire, causes him to suffer from Multiple Personality Disorder.

King the Land - Chaebol's panic attacks are triggered by [WTF?] insincere smiles that have something to do with his being abandoned as a child.

Protect the Boss - Secretary helps her demanding chaebol boss overcome his agoraphobia, tied to—you guessed it—a childhood trauma. The childhood trauma of the obnoxious chaebol in Secret Garden causes the opposite problem—claustrophobia—as well as memory loss.