Lee Seong-Chul is the Director of the Ilsin Hospital, the hospital where Park Kyung-Hye, Jin-Woo's mother, stayed when she was ill with Eternal Slumber. Although he appears in the webtoon, he is only named in the novel.
When Kyung-Hye suddenly woke up from Eternal Slumber after four years in a coma, Lee initially refused to let her go home so that he could figure out exactly how she was cured. However, after learning that her son Jin-Woo was a S-Rank Hunter, Lee immediately reversed course and allowed her to be discharged. It's also implied that he chose not to inform the medical community about Kyung-Hye's miraculous recovery, as Yoo Myung-Han and his staff had to do some digging to find out that she was cured.