Shannon is a Faerie that was placed in the crib of a human child who in turn taken by fairies, thus making her a changeling. She was raised by humans and grew up to become a doctor. She later returned to the Anthill after Shanahan, the human child she switched place with, found her and they later married one another.
Skills and Abilities
Mahou Tsukai no Yome Official Guidebook: Merkmal (p.48), Shannon's age.