What Could a Puppy Possibly Be Up To?

If not for that accident, ER-1213 should have transmigrated into a book as the heroine's younger sister, a competent tool character whose job was to move the plot along. But now everything has gone haywire. She has become the heroine's pet dog, And the one who took her place as the younger sister is a male from a nonhuman race... This situation... If the world falls apart and the protagonists turn evil, it definitely won't be her fault. After all, what bad intentions could a little dog have? P.S. Transmigrate into a book, both the male and female leads transmigrate, gender-bending, and different species. Original title: After Transmigrating into an Old-School Romance Theme: Return to old Earth and collect traces of civilization