Yun Woo (Family by Choice/Brewing Love)
According to his Asianwiki page, he was born on January 24, 1999. In 2020, he was first appeared on K-drama The Cursed. So far, he had six (6) Kdramas and even his debut film this year Uprising.

There are so many!! Too many to list haha 😅 But I'm always thrilled to see Im Chul-soo, Kang Ki-young, and Lee Si-eon. They always make me laugh!

Tough one.

Park Jin Joo - I wish to see her in a role where she sings.
Ha Do Gwon - I love him! He steals every scene he is in.
Kim Sang Ho - another good actor with strong presence.

Jo Han-Chul : I think I watched him at least in 5 dramas in 2024.

Jang Young-Nam : I really like her voice.

Kim Hae-Sook : she's not the grandma who always died. She really had different roles.

I'll be listing mine with where or when our relationship started.

Im Chul-soo (Since Alchemy of Souls)
Kim Tae-woo (Since The Spies Who Loved Me). I almost watched The Cleaning Lady because his name came up in the cast listed in the DB premiere recap.
Choi Young-joon. I formed this personal bias about him in Our Blues. I've loved him ever since.
Jang Young-nam (I can't recall where from)
Kim Sun-young. (Her Private Life). She was so hilarious here. I watched the first and only episode of My Lovely Liar just so I can see her act and yes! It was just as the recapper praised.
Kim Sung-oh (Since Unlock My Boss)
Hwang Bo-ra (What's Wrong With Secretary Kim)
Kang Mal-guem (Thirty Nine)
Kang Ki-young (What's Wrong With Secretary Kim)
Kim Ji-hyun (Thirty Nine)
Kim Sung-kyun (Sacred Divorce)
Ji Seung-hyun (You Are My Spring)
Jo Bok-rae (Taxi Driver 1)
Jung Woong-in. He is an oldie that I can't recall where I became so glad to see him cause I knew he'll bring his A-game, but the best I recall is My Sassy Girl/Empress Ki
Whee Hyee-hyang (Marriage Contract)
Lee Gyu-han (She Would Never Know)
Wang Bit-na (She Would Never Know).

Nearly all of our veteran actors and actresses are topnotch scene stealers! So I will go for younger ones.

Jang Hee Jin
The queen in The Red Sleeve, the sister in Flower of Evil, etc.
I've been excited to see her ever since I first saw her in Hello, My Teacher 2005.

Park Jin Joo.
Obviously.

Kim Ga Eun.
Da Eul in King the Land. She caught my eye in 2011, and keeps getting better and better.

Park So Jin.
Ki Ppeum in Shooting Stars. And I have just this second discovered that she is an idol and a bandmate of Hyeri!!!! What!!!! What rock have I been living under!!

So many good ones! The first three that come to mind:
Jo Woo-jin - he can be a pretty main character but never the main character (unfortunately) but he always plays interesting characters that are often intriguingly ambiguous. Whenever I see him, I know I won't be bored.
Im Chul-soo - a national treasure as far as I'm concerned. He can be menacing or pathetic or both of them at once, but he's somehow always hilarious!
Lee Seung-joon - a man who disappears entirely into his roles. When I watch him be a horrible abusive father, I can't remember him as anything else. When I watch him as a desperate Emperor Gojong who has nothing but bad options, I think it's all he's ever played. This man is so incredibly reliable!

Im Chul-soo
Kim Won-hae
Kim Sun-young
Kim Seul-gi 🥲
Park Jin-joo

Yes to Kim Sun-Young! And I’m always happy to see Kim Won-Hae.

Kwak Dong Yeon. For whatever reason, he doesnt get cast in lead roles as much so I always get excited when he cameos or is a secondary/supporting character in a drama. He is a great actor that makes his characters memorable.

If I spy Jo Han-chul, I know he'll always deliver a great performance and a memorable character no matter who this may be. His acting range is crazy.

Choi Moo Chul
Jang Young Nam
Jo Han Chul
Lee Bong Ryul
Kim Young Ok
Lee El
Kim Chang Wan
Sung Joon
Jeon Hye Jin

There's too many. Just yesterday watching Love Your Enemy I was more excited when they introduced the secretary (Kim Hyun Mok) and teacher Maeng (Jeong Hye Jin) than when I saw the leads.

Maybe it's because you don't know who's going to appear in the show (since the supporting cast doesn't appear in the promos pics and stuff most of the time), but when they appear on the screen it feels like a nice surprise reunion with your high school friends or something.

Ye Ji Won (where are you, bestie?)
Seo Jung Yeon
Kim Mi Kyung
Kim Won Hae
Kang Ki Young & Kim Seul Gi (still waiting for them to be the leads of the most hilarious romcom ever)
Sung Dong Il
Ra Mi Ran
Lee El
Hwang Bo Ra
Na Young Hee
Lee Mi Suk
Im Chul Soo
Lee Joo Seung
Jang Young Nam
Hwang Seok Jeong
Kim Min Seok
Joo Hyun Young
Kim Hae Sook
Kim Shin Rok
Song Ok Sook
Lee Il Hwa
Kim Young Ok
Kim Hong Pa
Jung Woong Un
Kim Kwang Gyu
Kim Hye Eun
Go Gyu Pil
Kim Yun Jeong
Jin Joo
Kwak Dong Yeon
Heo Jun Seok
Kang Shin Il
Shin Dong Mi
Kim Jung Jin
Baek Hyun Jin (I love how he plays THE insufferable guy everywhere)

...etc.

Some of them:

Kang Hoon: https://mydramalist.com/people/18814-kang-hoon

Shin Jae Ha: https://mydramalist.com/people/7436-shin-jae-ha

Kwon Yul: https://mydramalist.com/people/1272-kwon-yool

Joo Jong Hyuk: https://mydramalist.com/people/29937-joo-jong-hyuk

Ha Yoon Kyung: https://mydramalist.com/people/9536-ha-yoon-kyung

Jung Moon Sung: https://mydramalist.com/people/5670-jung-moon-sung

Lee Ji Hoon: https://mydramalist.com/people/4755-lee-ji-hoon

Seo Ji Hoon: https://mydramalist.com/people/12436-seo-ji-hoon

Seo Young Joo: https://mydramalist.com/people/2561-seo-young-joo

Choi Won Young: https://mydramalist.com/people/1209-choi-won-young

Min Sung Wook: https://mydramalist.com/people/3977-min-sung-wook

Seo Young Hee: https://mydramalist.com/people/1427-seo-young-hee

Hwang Young Hee: https://mydramalist.com/people/3267-hwang-young-hee

Oh Jung Se: https://mydramalist.com/people/4684-oh-jung-se

Park So Jin: https://mydramalist.com/people/8080-so-jin

Yoon Jong Hoon: https://mydramalist.com/people/5325-yoon-jong-hoon

Lee Tae Sun: https://mydramalist.com/people/11733-lee-tae-sun

Oh Eui Shik: https://mydramalist.com/people/10197-oh-eui-shik

Choi Moo Sung: https://mydramalist.com/people/3311-choi-moo-sung

Some of them:

Kang Hoon.
Shin Jae Ha.
Kwon Yul.
Joo Jong Hyuk.
Ha Yoon Kyung.
Jung Moon Sung.
Lee Ji Hoon.
Seo Ji Hoon.
Seo Young Joo.
Choi Won Young.
Min Sung Wook.
Seo Young Hee.
Hwang Young Hee.
Oh Jung Se.
Park So Jin.
Yoon Jong Hoon.
Lee Tae Sun.
Oh Eui Shik.
Choi Moo Sung.

(I had sent this comment adding links for all the actors, but it got stuck for moderation 😅).

Hands 100% down Baek Ji-won. Wherever she goes, I at least try and follow.

Even, oh yes, The Undateables. I'm 90% sure that's the one where she gets super-drunk starts hitting on guys she shouldn't and then hops like a bunny down the street...and if it isn't...it was one of the other 100 million great shows she made better by her screen presence.

Happy energy, sad energy, secret energy...she's got it all.

Im Chul-soo was my first thought here - since I saw him in Sweet mobsters (were he kind of ended up being the only interesting character for me), I have noticed him in things I have seen before and after, and he is just marvellous, always.
I have had the feeling of like, a gasp of happiness and expectation that things will be exciting now, for a long time with Dongha, apart from my many, many gifs about him, I made one just about that feeling. He is hiding from me here: https://dramabeans.com/members/CecilieDK/activity/1479594/
I like Kim Sun-young a lot, too. It's lovely to see her in such a nuanced role as in Virtuous Business! I have liked her many "The Tough Neighbour Lady" roles too.
Park Jin-joo shines so much in her roleas Nurse Oh in Jealousy Incarnate, that I made a long post about that; It's like she can be so, so funny with just a sceptical look and maybe a few words. She has done that a bit in It's Okay .... and also in Encounter, but to me, the smaller role where most of what she says is between the lines are the funnier.

Kim Mi-kyung (Healer, Another Oh Hae-young)
Park Jin-joo (Jealousy Incarnate, Her Private Life)
Hwang Bo-ra (My Girl, Arang and the Magistrate, What's Wrong with Secretary Kim)
Youn Yuh-joung (Ruler of Your Own World, Listen to My Heart, Pachinko, King 2 Hearts...)
Kim Kap-soo (Marriage Not Dating, Blade Man)

Actors I'm always happy to see unexpectedly and hope to continue seeing on screen.. (apologies to any that are considered leads in film or dramas I haven't seen):
Choi Dae Hoon
Choi Moo Sung
Ahn Jae Hong
Park Ho San
Oh Jung Se
Moon So Ri
Park Jin Joo
Kim Sun Young
Kim Kyung Nam
Woo Do Hwan

Kim Chang-Wan
Kim Byung-Chul

I've always loved character actors, to the point that I tracked down two out-of-print encyclopedias about all those Hollywood actors with the familiar faces and unfamiliar names. Since supporting actors are usually not mentioned in promotional materials, it's a happy surprise to watch a show and have a beloved face unexpectedly appear before me. I can name at least a dozen Korean character actors I'm always delighted to see.

I've seen him in leading roles in films, but the great Kwon Hae-Hyo seems to have been largely relegated to supporting roles in drama series. I have watched many an indifferent show only because he's in it. I'd put the brilliant actresses Kim Mi-Kyung, Lee Jung-Eun, and Ra Mi-Ran in the same category.

Other favorites:
Ahn Ne-Sang
Choi Won-Young
Kim Sun-Young
Kim Young-Min
Lee Mi-Sook
Oh Jung-Se
Park Ho-San
Park Hyuk-Kwon
Seo Yi-Sook
Sung Dong-Il
Yum Hye-Ran

So many talented artists

Im Hyun Shik
Lee Moon Shik
Ahn Suk Hwan
Song Ok Sook
Yoon Yoo Sun
Kim Mi Kyung

Kim Mi Kyung

My #1 fave is Im Chul Soo - just an amazing character actor. He never disappoints!

ACTORS
Kang Hoon
Kim Bum
Kim Hyun Mok
Lee Tae Sun
Yang Kyung Won

ACTRESSES
Gong Min Jung
Jang Hye Jin
Joo Hyun Young
Kim Mi Kyung
Kim Sun Young
Park Jin Joo
Park So Jin

When I see Kim Hae Sook in the cast I know I am in for a good time.

All of them !
Supporting characters are a major reason why I never like to leave kdramaland.

There is no lack of great supporting actors in S. Korea's film industry and no doubt many fine directors and screenwriters as well. From child actors to those still acting despite their old age, the only thing preventing them from being actively employed is due to the limited number of dramas/films that can be produced annually. S. Korea has more than enough fantastically talented individuals, more than even China. So much talent wasted sadly. But I digress and I agree with all the names listed here but no one mentioned Gong Min-Jeung. She has appeared in supporting roles in many films, TV dramas etc., plus a few main roles as well.

https://mydramalist.com/people/6104-kim-e-jung

Kim Sun Young
Cha Chung Hwa
Because of Crash Landing on You

Kang Ki-young
In his support role back then

Yang Kyung Won
Love him in Crash Landing on You and he just so evil in Judge From Hell

Lee El
In her support role back then

Kim Hae Sook
Mom or Grandma

Seo Jung Yeon
Since DotS

Park Jin Joo
Please back to dramaland

There's many more but I can't remember it now