A watcher is a simple type of servant spirit. Some experts consider watchers to be a tangible expression of the magician's own consciousness, molded from the fabric of astral space.
Assuming that this is true, and that your character could, indeed, summon a watcher spirit, what would it look like? What would it sound like? What idiosyncrasies would it have that would set it apart from other people's watcher spirits?
It would probably look vaguely like a small Chinese dragon or winged serpent, depending on what light you looked at it in. It would be a bit lower pitched and slower than the stereotypical high-pitched, hyper watcher, but just as excited to please. As far as idiosyncrasies, I'd say that it would probably speak Portuguese with a strong Sao Paulo accent and tend to utter the Portuguese equivalent of "ummm..." a whole lot.
(I don't really know if spirits use languages per say, but that sounded good to me.)