The Emperor is an unfortunate relic of the Daisy plotline, where they were almost certainly the Absolute looking to free itself from the control of the stones, and it's possible that an ending would've involved you 'dreaming down by the river' inside the hivemind after transformation. As it is, The Emperor inherited a lot of those aspects in a half-hearted, oddly implemented way. Why didn't he come to us as Balduran, for example? Because Larian didn't want to scrap the "dream waifu" aspect.
Tbf, it makes more sense for him to assume dream waifu form than his original form because he can read your character's mind and is trying to manipulate them. Showing up as his old self wouldn't necessarily have the same effect unless your character was into human dudes.
