Your character gains a secondary form, or transforms permanently. This form resembles their normal form, but they gain devilish traits. You can pick and choose any of these traits to apply to their devil form for free, or you can purchase and apply any non themed traits to appear in this form only:
For an example of a devil form, look at Agnes.
You can change your character's palette, as well as optionally change your character's markings to any of these: stripes, ember “flecks”, light freckling