Although what our player said above is true about some characters, I'm not sure if the same would apply to Hakkai due to the fact that he spends the majority of his canon in his "human" form, (while a character like Kurt is permanently "inhuman" in canon, and Rapunzel's change at night is fairly minor).

That said, I think the best solution would be for Hakkai to still have the ear cuffs in-game (as "sentimental jewelry"), or to strip him of his demonic powers altogether. In the case of the former, you'd have to explain/clarify in your power-limiting section how you plan on limiting his demon form, how it'd be different in the setting of Landel's, etc.

Hope that helps!