Straight: Emperor Evulz, after initially fighting Alice and Bob in a demon-like form, transforms into one that looks like an angelic version of his normal body. Naturally, this proves to be even tougher than his demon form.
Throughout the story, Emperor Evulz went through many forms, each are stronger and more monstrous than previous. His finally form however, is an incredible handsome lad yet far more powerful than everything before.
Evulz looks like a Rubber Forehead Alien until he ascends to his most powerful form in which he simply loses the ridges on his forehead.
Evulz' final form is still a monster, but more human-looking than the last form.
Evulz' final human-looking form is not stronger but it does have abilities that outnumber its weaknesses.
Emperor Evulz is a Master of Illusion, and the demon form was a physical projection while the real him prepared for another go at Alice and Bob. After the demon's defeat, he has both learned their tactics and has better equipment (the "wings" of his angel form are Magitek armor).
Emperor Evulz's real form is the angel, and he was keeping the demon form up through sheer strength of will. Now that it's dead, he can focus his full power into combat.
Emperor Evulz is embodiment of evil, each of his monster forms reflect how repulsive evil is. His final form, however, reflect the source of evil itself.
Emperor Evulz's pretty angel form makes him a much smaller target and allows him to move much faster than he could as a giant demon with decidedly un-aerodynamic oversized horns. As such, he's harder to hit, and his attacks are harder to dodge. The bright, shiny sparkle surrounding him and the light radiating from his body also make it that much harder to look at him for very long.
Played For laughs: "Watch out! He's morphing into his most powerful form. Whatever you see, don't turn back! He have to keep ... Wow, he's hot!"
Played For Drama: The heroes not only have to face Evulz but now they see his most powerful form is that of a handsome man. They were oddly comforted when he was an ugly monster but to see such a horrible being with the face of a man disturbs them as they see a dark mockery of the worst of humanity.