All great ideas for Kristoff. I also like the idea of him using force-field armor. The Machiavellian personality would also work well for Kristoff. Him using all of his resources (magical, technological, political) allows for him to fit in a number of stories. His intelligence allows him to act as a foe or even an ally to just about any character.
What do you think would be a suitable code-name for Kristoff? I think he would need a firm identity/moniker; one that allows for him to further develop a personality of his own.