Ja Gwi was a subordinate of Bi Gak. His reputation among other Chachaoong was very bad and he had been in hiding for years for fear Chau Yoong would find and kill him. He was mostly known for finding young Chachaoong and maiming them in some way (such as removing their eyes or ability to speak like Hil Jo and Ha Je) then placing deadly mark on their body, forcing them to serve him. Most of his servants died in horrible ways, according to Hil Jo, and it was implied Ja Gwi was exceptionally cruel to them. Any who disobeyed had their marks activated, and were killed.
It's unknown what his goals actually were, but he was finally stopped after an awakened Yu Jin removed his left arm and sent him running. After being found by Chau Yoong, he was taken away to be tormented for his crimes as killing young Chachaoong is against Jack's rules.