Jean-Guillaume is Kaeloo's psychotherapist.
As the show never shows any characters apart from the main and secondary characters, Jean-Guillaume is always off-screen, and as a result is never seen.
Role in the show
He is Kaeloo's psychotherapist, and she frequently calls him and asks for advice, especially on the subject of controlling her transformations.
He had a major role in "Let's Play Jump Rope", where he had to psychoanalyze Mr. Cat (to try and find out the reason for his cruel behavior), and at the end of the episode he had to psychoanalyze all the characters except Kaeloo herself and Olaf.
In "Let's Play to Win", Kaeloo declares that she no longer requires his services. However, in future episodes, she appears to have returned to being his patient.