Sadly, I knew The Imprisoned was Demise because I spoiled myself early on. :<
But when the game finally arrived at my house and I got into it, the details still weren't all that evident to me. The Imprisoned emerged from his seal, but I didn't see how he directly tied to Demise. The story overall provided very little hints until his resurrection. Meanwhile, each time The Imprisoned returned more doubts formed in my mind that they had any connections. I thought that, despite their comparable body features such as the scaly hide, The Imprisoned was just a guardian of Demise's prison, but he was trying to escape in order that he could break Demise from his bonds (as I imagined he was chained up somewhere in the portal) by attacking the Sealed Temple.
It became all too clear that I was mistaken the more that Link went to the World of the Past. That must be Demise's true state of imprisonment, was my immediate interpretation as I scanned the outside of the Past Sealed Temple for the first time. At the time I also realized that they were not two separate entities, but the same creature yet were changed over time, to my best understanding.