Guy finds a hurt Tanuki in the forest outside of his town. He takes it in, nurses it back to health, then tries to let it back into the forest. But the Tanuki doesn't want to leave. He decides to let it stay with him as a pet.

A few days later the school year starts. And when he arrives, there's a new girl there with giant breasts and she declares her love for him. He refuses, not even knowing who the girl is or what's going on. But despite his rejection, she proceeds to follow him everywhere, hug him at random times, follow him home, etc. To the point that he can't go or do anything without her around. It starts to bug him and he's constantly trying to find a way to get away from her. His only solace is when he goes home and gets to spend time with his Tanuki pet. Which the Tanuki will crawl around on him and stuff. But he doesn't mind since he expects that from a pet (and he prefers animals to humans).

Obviously the girl is the Tanuki who transformed into a girl. But she has to hide her true identity because of his reaction to her human form. She has to always be around him so he doesn't find out her true form.

tl;dr 1980s style romcom with a Tanuki girl.