This is a term I hadn't heard of before I began watching kdramas, but it happens quite often: the second lead is more appealing than the lead actor. This usually means that you ship the female lead and the second male lead. (Shipping means you root for a relationship.) For example, in Boys over Flowers why didn't the girl like Blondie, who was much nicer to her than Curly Hair? He was more her friend, and you'd think she wouldn't have hung out with him so much if she didn't like him. Although he couldn't act. But Curly Hair Guy had intensity going for him.
In Sungkyunkwan Scandal (a story about a girl dressing up as a boy to get into a boys' school) although the two main characters did get along together well, I still liked Robin Hood better than Rooftop. He was mysterious, a tough fighter, and above all, possessor of The Mane of Glory, which is supposed to be a sign of the hero. He was smart; he had read all the books in the library. Besides that, he was often to be found hiding up on top of a roof or posing tastefully in a tree.
A gumiho is a nine-tailed fox from Korean mythology that can turn into a person. The one in My Girlfriend is a Gumiho falls in love with a human and wants to become human and stay with him. A half-goblin man (who is much more beautiful than anyone else) is sent to catch her and decides to help instead. You wonder if she will stay with him or the human.
Greatest Love is another one. An arrogant, very famous actor thinks he loves a has-been singer because when he hears her sing his heart beats faster. The real reason is that he heard her songs while having heart surgery (however logical that is). The singer meets a cute acupuncture doctor and has to choose between them. The doctor is nicer than the actor and moreover doesn't have an evil weird cackle.
Prosecutor Princess is about a fashion-crazy prosecutor gradually getting to be effective at her job, being coached along by a (seemingly) friendly lawyer. Nampyeon and I liked a male prosecutor (the Reaper from Arang and the Magistrate) as much as the lawyer, and the Princess has competition from another female prosecutor who also likes him.
In Secret Garden the male lead was pretty mean for much of the show, although he became nice later on. It was our first kdrama and we didn't know that this is a common plot ploy, so we resented it more than we would now. When the director of the stunt school seemed better than Rich Boy, Second Lead Syndrome hit. He was reticent but kindhearted. Then Rich Boy's cousin began to grow on you and SLS hit again. Oof. Twice in one show.
In general I like lots of good secondary characters, and I like more good guys than bad guys. But do they all have to like the same girl? Sometimes everyone pairs up and they are all happy, but a lot of times they don't. There's just a little tendency towards melancholy in Korean dramas. But who knows what will happen? The second lead in You're Beautiful didn't get the girl, but they paired up later in Heartstrings and got a second chance. Although it turned out a little dull. Now I need another show with Cousin Oska from Secret Garden in it.