My theory is that Jooin suffered an accident with/because of Cain as a child so now he doesn't remember a part of his childhood. That's the reason he doesn't remember Yahwi nor Cain. Yahwi does not know about the accident but Cain feels responsible about what happened to Jooin that's why he follows him and calls him master. It's a cliché theory but who know might be the case ╮(￣▽￣)╭
My theory is that Jooin suffered an accident with/because of Cain as a child so now he doesn't remember a part of his childhood. That's the reason he doesn't remember Yahwi nor Cain. Yahwi does not know about the accident but Cain feels responsible about what happened to Jooin that's why he follows him and calls him master. It's a cliché theory but who know might be the case ╮(￣▽￣)╭