King2Hearts - She is probably my favorite second female lead. She's Lee Seung Gi's sister, so there's no competition for romance, and her story arc was really inspiring.
Pinocchio - She wasn't vindictive, and she didn't spend the entire drama trying to pursue Lee Jong Suk. Her scenes were often less serious, so I enjoyed her.
Pasta - She will spend some time pursuing the male lead because they have a history, but I liked to watch her journey as a chef in a male-dominated industry. She didn't back down as much as Gong Hyo Jin's character did. The male lead in this drama is pretty mean, though, fyi. Not sure if you have a preference there.
The Princess' Man - Her start was a bit rough (I really thought she would be like many other second female lead, but she grows a lot as a character and you can see her motivations. She'll complicate the romance between the leads at first, but her path leads her in a different direction.
Good luck in your search!