Who is Saturday from The Girl Who Circumnavigated Fairyland in a Ship of Her Own Making and what is their importance?

Blue skinned Saturday is a Marid, or water fairy, who has the power to grant wishes to anyone who subdues him in battle. He is captured and imprisoned by the Marquess, but freed by September, whom he accompanies on her journey through Autumn. He is quiet and soft spoken, with a voice like the whisper of the sea. At the conclusion of a series of dangerous adventures, Saturday (who can interact with the future, like all Marids), catches a glimpse of the daughter he and September will have. It is therefore reasonable to assume that it's Saturday to whom September loses her heart, as foretold by the choice she makes when she first sets foot in Fairyland.