I summon you gold wang 's family on air monstar
the one i can think of is "I summon you gold" the male lead Park Hyun-Soo doesn't end up with the female lead Jung Mong-Hee but ends up with her twin sister (plays actually by the same actress) and the female lead is alone at the end. thers's also enjoybla sides stories in this drama and its also about brotherhood.
On air Oh Seung-Ah played by kim ha neul and Lee Kyung-Min played by park yong ha are the main leads Lee Kyung-Min ends up with the second lead female Seo Young-Eun played by the song yoon a the main leads were never a couple but had a lot of interactions that made the second lead jealous.
another one played by actors a little old but still had its followers it's the couple in "wang 's family" Wang Soo-Bak and Ko Min-Joong are the main couple of this weekend drama but wang soo bak mistreats her husband and doesn't valorize the family she has with him so they will get a divorce and he will start a new family with his first love Oh Soon-Jung (i really liked this couple especially the daughter he didn't know he had with her Koo Mi-Ho )
Monstar the main female lead ends up with the second male lead and the first male lead sun woo at the last sceen gives a feeling that he started to think about the second female lead kim nana that's head over heels for him.
if you are interested in other coutries series as a male lead and second lead female there's a brazilian serie "a força do querer" jeiza and Zeca, a turkish "Aci Hayat " Mehmet Kosovalı and Feliz.