
Wait, its just a hunch but I feel like Cain was a dog but then reincarnated to a human. 1. He says he remembers being petted. 2. He calls Jooin master in a non kinky way. 3. I think when he visited Korea, he was staring at some building (perhaps it used to be his old home?)

Plot twist!!
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- Note: Keep in mind Cain actually speaks very fluently in Korean, but specifically hiding it from Jooin...
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Cain and Jooin, They have been in master and servant relationship in their previous lives, in historical periods. Jooin was noble family young master and Cain was his servant/bodyguard/knight in shining armour. Cain was foreign born and sold at a very young age to Cheoson (historical Korea name) and became Jooins servant and growing up together fall in love with him promising he will keep his master safe and protect him in all cost. And Yahwi was crown prince that also had a "THING" for Jooin and at the end was one of those who hurts Jooin s feelings and was the reason Jooin died. And Cain also left heartbroken for not being able to protect Jooin from death.
So being reborn/reincarnated Cain remembers their past life and after seeing Yanhwi together with Jooin on abroad, I mean in hotel, he decides to follow Jooin to Korea as he knew that same things as in past may happen. And negative things actually start to happen.
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And last plot twist:
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This is just my GUESS not an actual story lol. ╮( ̄▽ ̄)╭
Sakuna expirence the ultimate fanboy/,simp