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It’s been emphasized several times that the prince is a very skeptical person trying his best to survive in the midst of a political struggle. His knight is literally from a family that wants to off him. It would be out of character if the prince decides to trust Kaisel and be nice to him after this one assasination attempt that for all he knows could have been planned by Kaisel’s fam ( ̄へ ̄)
Honestly can’t blame for only seeing Brian through Ian. They were so in love so I could only imagine Sol’s regret was immense since he was too late to save Brian. Hence he cannot fathom to see Ian as a “new” and “different” because he will need to accept that Brian is gone :(