He is a member of an independent activist group for Korea. After one mission goes awry, Ui-Hyeon is shot, falls into the sea, and is rescued by Su-a. 

The year is 1926 in this retelling of Hans Christian Andersen’s “The Little Mermaid”, and Joseon (present-day Korea), is under colonial Japanese rule. Su-A Heo is a handmaiden who spends her days working and swimming in the sea. Her work is her whole life, until one day, when she discovers a wounded independence fighter named Ui-Hyeon Kang washed up on the shore. She nurses him back to health, only to lose her ability to speak in a botched murder attempt by Ui-Hyeon’s comrade, Hae-Su Song. As she travels to the capital, Gyeongseong (present-day Seoul), to get revenge, she finds herself growing more and more intertwined with a strange, brave new world and the people within it. (Source: WEBTOONS)
 
Note: Chapter count includes prologue.