Tang Seoyoon (I Ended Up in the World of Murim) manhwa | webtoon

Tang Seoyoon, a cold-hearted woman and a poison expert that serves The Poison King and his wife Un Gareo. The MC, Jang Sam a.k.a. Jang Sunwoo meets Tang Saeyoon and he makes a deal with her to train him to be able to resist poison. She opens up to Jang Sunwoo after her brother dies in battle. Although she appears early in the story, she is the final member to join Jang Sunwoo's harem.