The earliest book I ever wrote was The Painting. Adapted from an unfinished comic I also made, the story followed young Elizabeth (Lizzy), the daughter of two artists/art museum owners, and her cat Bast.

The museum had recently received a new shipment of art, stored in the back room. Lizzy brings Bast to wander in the museum. The cat accidentally enters the room and unveils a cursed painting -- freeing the unnamed Angel-Demon who traps the cat into the painting. Lizzy, who was searching for Bast at the time, is sucked into a portal created by the Angel-Demon and sent to the Art World.

Lizzy wakes up in the Art World with a talking cat who looks like Bast but claims not to be. She is told she is one of the two legendary saviors who will defeat the Angel-Demon by collecting letters of its name across the Art World to unlock its secret cavern.

Lizzy: Tall, athletic young protagonist (no older than 12, though I can't remember her exact age). Blond with bangs. Not the smartest, but knows a lot about and loves art. Weapon is sword.
Bast/Isis: Fluffy grey cat. Has a lot of sass. Acts as a guide for Lizzy to navigate the Art World.
Ben: Side character and another savior, weilds a scythe (he is Dark, Lizzy is Light). Apparently they know each other from French class in school, but Lizzy has never spoken to or noticed him.
Anny (Alt): A member of the mysterious Huntre clan, saves Lizzy from an attacking serpente.
Codi (Alt): A member of the Paintre clan who sketches Lizzy a bridge to avoid battling a troll (?).