This is a good topic I read the first part and started to compose the characters I hated then saw the end of the sentence and could not think of anyone apart from Yoorim. I did not like how she treated Heedo and they did a great job to make me change my mind. Everyone else it has been too little too late.
So I am pulling up a chair and will wait to see what the other Beanies bring to the table.🍿🍿

My picks:
* SEO Do-yeon in 'I Can Hear Your Voice', the FL's former high school classmate / rival attorney
* SONG Ka-kyung in 'Search WWW' - the FL's colleague turned competitor

Jun-ho’s character from Chief Kim

Oh, this is such an interesting one! I think these are going to be somewhat subjective, because people like/dislike characters for different reasons, and have different things that are deal-breakers for them. Here's my best shot:

In My Ajusshi, I think I would put Dong-hoon's wife, Yoon-hee, in this category. I had a lot of disdain for her in the beginning (so many poor choices!) but I grew to understand her a lot more as the show went on, she was legitimately sorry for the choices she had made, and then she went on to be a source of support for Dong-hoon and Ji-an.

Another might be Lee Jun from Chocolate. I have a soft spot for Jang Seung-jo anyway, which might have something to do with this, but I never totally disliked him in the beginning, though he is a real jerk. But again, the story humanizes him over time and you learn more about his terrible family and why he acts the way he does, and then he starts to question why he's been in this hyper competitive relationship for most of his life and sort out what is actually important to him. It was one of those where you could see his redemption arc starting from early on, and I was so ready for it.

Maybe Jung Yoo-taek, from Just Between Lovers/Rain or Shine? I don't know if he ever gets to the point of "loveable" but he does start off extremely unlikable haha! And - same story - you realize that he's just super afraid and in a job he can't handle and extremely lonely. I don't know if I exactly liked him so much by the end, but I did like his relationship with Ma-ri, and that definitely counts for something.

Possibly a more unusual one: Cho Beom-pal from Kingdom. Wow is he ever annoying/incompetent/neglectful of his duties at the beginning, and he honestly stays pretty cowardly throughout (though, um, understandable). But his infatuation with Seo-bi (as is often the case for drama characters) proves the saving of him, and by the end he is making at least some better choices and I found him pretty endearing.

Hum... It's funny because I didn't hate Yoo-Rim neither Min-Woo (I didn't really care about PD Kwon Young-bae in Today's Webtoon.). Neither of them were bad people but they did make bad choice or acted wrong but this kind of character is more interesting than the ones who always are perfect. Min-Woo was more interesting as a character than the ML in EAW. The ML was very swoony but it's all... No devolpment for him. For Min-Woo, we could understand why he acted like he did or why he changed. It was the same for Yoo-Rim, she was just a teenager after all.

Hate is too much but I would choose Baek-Hyung in Extraordinary You. A lot of people said he was a jerk because of the writer and it was partially just but not completely, he was a jerk even in the shadow scenes. But at the end, when he could have what he wants, he knows that it would be a lie and take the good decision.

Na Ji-Hae in Suspicious Partner, she was the one he cheated on the FL with, she made rumors, etc. But at the end, she was the one who lent her couch to the FL because she didn't have any home.

One whose redemption arc didn't work for me was Han Se-Kwon (Lee Sang-Yeob) in On the Verge of Insanity. He didn't deserve his wife!

The drama that came to mind when I read this question was Haechi. While I wouldn't call any of the characters lovable, the theme of redemption, growth and change of the characters was woven through the drama. So unlikable characters become likable, forgivable or at least understandable given the life circumstances. It has that believable redemption arc that is hard to find done well. I thought it was a pretty great drama especially if you like Saeguks.

Kwak Dong Yun in My Strange Hero

Lee Jun Hyuk in City Hall

Jo Han Sun in Stove League

Lee Sung Min in Miss Korea

Kim Hae Sook in Marriage not Dating

Ryu Duk Hwan in Special Labor Inspector Jo

Most of these were the first role I saw them in and made me fell for them HARD.

The ML’s mom in Marriage Not Dating. Never thought I’d love her by the end.

Oh! The character to take first place is Go Hyunjung's Mishil in Queen Seondeok. Mishil was characterized as the character we'd hate since she provided great opposition to our leads and their endeavors, but she became the reason I sat through Queen Seondeok.

The most recent will be In-hye in Little Women. Dang this character grated my screen the first 4 episodes with her child adult behavior but she's become the sister I like more. She's overthrown In-kyung.

Im Ji-kyu in Her Private Life is the only Im Ji Kyu I came to really like. The others - stellar acting from Im Ji Kyu but I just couldn't reconcile with them. Their sins were too much, beginning with his character in Gwangaetto, the Great Conqueror. (P.S. His scenes are really small.)

Lee Hyun-Wook in Mine. Even when his bad side were unfolded the more after his passing, I just ended up understanding him and liking him more. If you're looking for a redemption even in death, you'll get it.

Honorable Mention.
Chu Ja-hyun : My Unfamiliar Family

Sung Bo-ra from Reply 1988. Can’t think of another character that fits the description this well.

I feel your pain. I will always hate the dude from Woo. Another character development failure it's the "friend" from Flower of Evil, imo.

—Five Children (Jin Joo's character development is so cool. She's my favorite character of the show)
—My Husband got a Family (big sis -she ended up being my favorite character too)
—Terius Behind Me (brother~~)
—Protect the Boss (second female lead)
—School 2013 (the teacher -Choi Daniel-, one of the bullies -the one that wanted to go to college)

not a Kdrama, but all the characters surrounding Ted Lasso!
they all start out as seemingly jerks, and you end up loving all of them!!

Sassy Go Go STILL has one of the best redemption arcs in kdrama I've ever seen and yes I will continue to promote it for that.
So Kwon Soo Ah played by Chae Soo Bin in Sassy Go Go. It does take the entire show to get there, but that's the point. I love her arc so much. I love Sassy Go Go in general so much tbh.

Others:
Similarly, A Moment at 18 does the same thing with its antagonist character, who's name I've forgotten, but not nearly as strongly as SGG.

Do Han from Lookout/The Guardians SORT of counts? I think he counts but he's also more of an anti hero ish character from start to finish and I don't know if he really becomes traditionally likable by the end. I love him though 🤭

Second @midnight's submission of Kwak Dong Yeon's Character in My Strange Hero and Ryu Duk Hwan's char in Special Labor Inspector Jo.
Actually MSH all around does this pretty well.

Lee Rang in Tale of the Nine Tailed (😭😭😭)

Kim Kyung Nam's characters in both Strongest Delivery Man and Come And Hug Me both have interesting arcs along these lines.

I mean TECHNICALLY Bead Boy DID get his redemption arc in Secret Royal Inspector and Joy but that show didn't deserve him and also Bead Boy 😭😭😭 (id est this is not a proper recc)

Not a drama but obviously Zuko in Avatar: The Last Airbender 🤣

What category do the kids from White Christmas and Extracurricular go in? Probably not this one haha.

Now ask me what shows SHOULD'VE had redemption arcs but didn't 😂😂😂

I suspect I will not say Jun-kyung from 'if you wish upon me'

Seo Dong-jae

Jo Jung Suk character in Jealousy Incarnate (Dont Dare to Dream). He's so annoying I almost dropped the show after 1st episode. I'm glad i didn't because I grew to like him and the show.

Lee El's character in Liar Game is one of my personal favorites.

Cho Sang Gu in Move to Heaven

I fear that I’m always failing to answer the actual question because my mind decides to think to the left of the point, but I wanted to say that one of the reasons I liked Love Between Fairy and Devil (Cdrama) is because they made the ‘villains’ so interesting and human. Instead of having characters who are easy to hate, we have characters who are deluded, who make selfish choices (to survive, or ‘for love’), who repeat the same mistakes despite seeing very well that they won’t arrive at a different outcome, or who walk on one side of what they deem is a very bright line between good and evil but who are ultimately mistaken about that dichotomy. Those are my favourite sort of villains because they feel so close it’s uncomfortable.

Joo seok kyung & Jenny: penthouse

Kang yeo seo : Sky castle

Kang Dong Jo and Do In Beon in Romantic Dr. Kim. The first probably have one of the best character development in a drama I’ve watched so far. Also Jung In Kwon in Our Blues. At first he is seems only bad tempered father completed with foul mouthed and violence tendencies. But after his story revealed, he seems so much human. I think it’s not per se I love him as human being, it’s the writing of his chatacter that I admire. The imperfection of his character is perfect.

a character type more than a specific character: second female lead when they are relatable and earnest. in Protect the Boss for example.

hmm...
and just, Bridal mask of course, Shunji.

Prince Won from Alchemy of Souls, hated his entitled assholiness in the beginning but now I’ll sacrifice my first born child for him

Also Yoon Baek-hee from Dream High, she was more tragic than anything, but I think it’s the first time where I fully grasped how too much insecurity can become toxic