So my guess is that Jooin probably knew both Yahwi and Cain when they were younger and somehow lost his memory and Cain probably had something to do with it (purely because he's a lil sus, and the way Cain and Yahwi act is also sus) . Cause that would explain how Cain knew Jooin's name when they met and created a fake story about him being his Master in another life just so he can get close to him again. But that's just a theory...
JAEYIIII! YOU BETTER LOCK IINNN!!! LOCK. IN!!!!