Celeste is Blathers' younger sister. Her character design is similar to his, except with the predominantly red color scheme, and a big pink bow on the back of her head.
She's had different roles in various games of the series; working in an observatory, managing a gift shop, or appearing to inform the player about nightly meteor showers. In most games, her role is themed on outer space, stars and other celestial objects. (Hence her name.)
This makes Blathers and Celeste the "owl scientist siblings" of Animal Crossing: The older brother skilled in biology, and the younger sister skilled in astrophysics.