I’m looking for kdramas where the ML is competent, but also has a good personality and is mature. I don’t like mean/cold/stoic ML, but I also don’t really like ML that are like spoiled and immature in a “funny way”.
Some examples of ML that I’m looking for are:
Vincenzo
Descendants of the sun
Suspicious Partner
Healer (competent and badass but also had a cute/good personality)
K2 (he had a cold attitude towards those who deserved it, but you could see since moment one he was a good, mature person)
Undercover High School (he’s funny, but definitely mature)

I’d prefer action and romance as genres, blut I also really liked the ML from:
Twenty-five Twenty-one
Welcome to Samdal-ri
Love scout (i actually have to yet finish it, but i really liked the ML)
See you in my 19th life (have watched it a while ago but I remember a decent nice personality)

Happiness - he's a cop who's super competent, brave but also just nice. Not immature or cold at all. Also, Happiness is so underrated, it's a great action/thriller kdrama

Doctor Slump  - same actor, he's also very likeable here, funny and not mean, although him and the FL have a bit of a cat and mouse thing going on in the beginning but he isn't mean. 

Love All Play - he's a golden retriever type of ML, super cute and super dedicated to the FL. I loved this Kdrama, but it gets angsty at times (which i personally love).

Not sure if you only want kdramas, but there's a taiwanese drama which has a cute and funny ML who isn't mean and he's also strong and smart: Someday or One Day