Lee Yeon-saeng(이연생 (李連生)) is a major character in Dae Jang Geum. She is the best friend of Seo Jang Geum. Since the first day they entered the palace as children, the two have always been there for each other. When Jang Geum is exiled to Jeju, Yeon-saeng weeps so much King Jungjong sees her and she doesn't even see him. Jungjong takes a liking for Yeon-saeng and makes her his concubine, despite Choi Seong-geum told him a negative remark against Yeon-saeng. When Jang Geum returns to the palace, Yeon-saeng becomes pregnant and becomes Lee Sook-won(Royal Concubine Lee)(이숙원 (李淑媛). She appoints her mentor Min Mee-geum and other friend Chang-ee as her personal assistants.
Yeon-saeng is portrayed as a sweet and caring but emotional and physically weak young woman.
For the second half of the series Yeonsaeng is known as Lee Suk-won. There was indeed a Lee Suk-won, who was one of Jungjong's consorts, who bore him 2 daughters, Princesses Jeongsun (정순옹주) and Hyojeong (효정옹주).
The role of Lee Yeon-saeng is played by the Korean actress Park Eun-hye (박은혜 (朴恩惠)), who was originally supposed to play the role of Seo Jang Geum.