His disinterest in things is in his blood, it's not every generation but there's been a few kings before him who were the same. They try to search for things that interest them and get obsessed to the point of self-ruin, like getting too invested in research and staying in their room or getting too into war tactics and just having continuos wars during their reign and stuff. I think there was a specific one, which might be the first king, who found a girl and was able to chill out, which connects to the game's story. Bertia believes the heroine to be the "chosen girl" (I forgot what it was actually called, but same concept) because in the game, in Cecil's route, the player saves him from his boredom and prevents his "ruin", which would be starting wars/having self-destructive tendencies. Which is why she's been pushing it toward the Cecil "good end", which would probably be the happiest ending for everyoneexcept her. So, not as exciting as a demon or anything, but still a not great ending. Just unfortunate that she's too caught up in the game's story that she doesn't realize that things are already different, which I guess is the norm with these stories lol