Aoxil was sold as a baby to be a pet. Not much was known about the species at the time, so he was simply sold as an exotic species without getting proper licensing or research. Celeste happened to be the purchaser. Given her time spent working at one of Rêve's possibly existing estates, she has picked up on his eye for all things special.
As Aoxil grew and matured, it was obvious that he had a the faculties of a more intelligent being, but he chose to allow himself to be kept as a pet. Celeste knows he is intelligent and he knows that she knows, but neither of them speak much of it. They do exchange words occasionally, but most of their relationship revolves around the howl's beautiful singing... and both Celeste and his libido. :x

Howl species and art (c) Lotix Lotix