I think he should be introduce at the end of Gehenna's world boss level and has unique interactions with every character you play as.
Example: "well well well, if it isn't everyone's favorite turncoat demon Firebrand. I am the Fusionist, and I see that you've been making quite a mess thru out my personal laboratory. What's a matter, still mad that I'm using your kind for my fusion experiments? I assure you, that your lord Loki agreed to lending his subjects to my cause so they ascend to much greater forms. And now Firebrand you will see the fruits of my research, my greatest creation yet that combines the power of both monsters and demons.
Other Example: "Why if it isn't the Mushroom Kingdom's favorite number two, Luigi. I am the Fusionist, and I'm impressed you've mustered up the courage to make it this far despite your cowardice. Those experiments you've faced are just minor footholds to the far more greater creations I have yet to unleash. And now little plumber, you will face the greatest creation I have in-store yet, a beast that combines both Birdo and Demon"
Regardless of which character you play as all interactions lead to the same outcome, with the Fusionist unleashing Cacobirdo and teleporting away. But not before saying "COME FORTH CACOBIRDO! Show these fools the might of our ascension, and the consequences of those who dare defy us!"
Like my advice? let me know.