Jay Ukyou
Resistance is Futile
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Maybe it's meant to show that the Mysterious Man can shapeshift in general?
Perhaps we've been looking at this the wrong way. The important part isn't the Digimon Kaiser. The important part is that we have a shapeshifter villain in the story. And that means he could be masquerading as just about anybody among the tertiary cast. Maybe this is just foreshadowing for a big reveal that Daigo has been the Mysterious Man all along?!
Then why take the form of the Kaiser at all? Not saying your wrong, but if the point of his changed appearance is that he's a shapeshifter, there's a vast number of things he could do better and more efficiently -- like replace Maki, for example, and exploit her position to further his own goals.
The idea of becoming the Kaiser would be to throw off the audience in this scenario, while still foreshadowing the shapeshifting ability so it doesn't 'come out of nowhere'. Also, the more likely candidates for disguise would probably be Meiko's father or mother.
It's just a thought. There really aren't enough non-Chosen Child human characters in the story to exploit this way for it to be a solid theory.