Her backstory is a bit of "A Clockwork Orange" with some of that D&D flair. As a convicted pickpocket, she was given the choice between serving hard time or submitting herself to an experimental (and quite controversial) criminal rehabilitation... "treatment." This procedure, conducted by an extremist sect of the Church of Pelor, placed a powerful geas on her. Now, she gets a little bit of a radiant "jolt" whenever she behaves badly. Vivian's a pretty rotten gal, but she can never seem to fully express that these days.
Her familiar, Grigori, is a nosy little Cherubim (a reskinned Pseudodragon) sent to keep tabs on her, making sure she doesn't misbehave.
Mechanically, Vivian's a multiclass character - specifically a Celestial Warlock, with a four level dip into Divine Sorcerer. This gives her a little bit of damage, a little support, and a few fun and tricksy utility abilities. Combining her class abilities with the Tiefling racial feat "Flames of Phlegethos," she gets bonuses for casting all sorts of fire and radiant spells - thematically perfect for a... "follower" of Pelor.