Sonia is the girlfriend of the main protagonist of the franchise, Heathcliff.
Sonja is a white cat, possibly an Angora. Often seen with a pink bow around her neck, and a pink ribbon on a single strand of hair on top of her head. She also has a little beauty mark under her left eye. The "whites" of her eyes are pale blue.
Sonja (in the comics) is loyal to Heathcliff to a great extent. She never cheats on him, but in the DiC animated series, sometimes guys like Pecks Beefcake (not to mention Hector!) tried to woo her, only for them to end up face-to-face with Heathcliff!
But she is not a pushover, either: In the episode Terrible Tammy, Sonja stepped in to fight Tammy because Heathcliff was neither going to fight nor back down from the bullying female feline, proving Sonja can fight just as furiously as Heathcliff himself!