Lydia is an overzealous animal lover who first appears in the episode "Fetch".
Lydia is a short adult human woman (even shorter than Star Butterfly) with tan freckled skin. She has long, somewhat bushy black and gray hair, and she wears a gray shirt, green jacket, long blue skirt, brown shoes, and glasses.
Lydia is outwardly eccentric and bears the stereotypical appearance of a "crazy cat lady", but she is very soft-spoken and affectionate. She also tends to hang around places where there are animals, such as behind the Stop & Slurp and in the dog park. Given how she said "This is what happiness feels like" when petting Willoughby, it can be assumed she is rather lonely.
In "Fetch", Lydia appears when Star answers a "Lost Dog" flyer for a lost "smallish medium-to-large dog" with a "dogish face" that "may or may not bark". When Star calls the number on the flyer and the cashier at the Stop & Slurp answers, Lydia appears outside the cashier's window to take the call.
When Star attempts to return to Lydia a stray dog that she believes to be Lydia's pet, Lydia gives the dog's name as Willoughby, having made it up based on seeing a street sign and a bumblebee. However, when Willoughby disregards Lydia to use the bathroom, Star realizes that Lydia is not Willoughby's owner.
Near the end of the episode, Star pairs Willoughby and Lydia together as pet and owner when Willoughby, revealed to be from another dimension, expresses a desire to be like a normal dog.