Prince Marvel is the human identity assumed by a fairy for a year's worth of adventures.
In form, Marvel resembles a youth about fifteen years old; he is handsome and manly, but not as large as a fully-grown adult male. Though his true fairy self is female, she has no trouble in adopting a male role — though Marvel never engages in courtship or other overtly romantic activity.
Marvel retains all her/his fairy knowledge and powers; indeed, she/he seems to be unusually powerful, rather than an "average" fairy (if such a concept is allowed). Marvel is "especial friends" with the Prince of the Knooks, the King of the Ryls, and the Governor of the Goblins; the Sound Elves, who carry messages with great rapidity, also fall within his orbit. (The fairy who becomes Prince Marvel is extremely old, and remembers when the Island of Yew arose from the sea in an earthquake. She has had plenty of time to cement influential relationships.)
Indeed, Marvel handles all the challenges of his year of human experience with aplomb; it is an open question whether he is ever in actual danger on any of his adventures. (The Enchanted Island of Yew)