Montmorency Margarita la Fère de Montmorency is a noble student at Tristain Academy of Magic, heir to the Montmorency family renowned for water magic expertise and their historic contract with the Spirit of Water at Lake Lagdorian. An encounter arranged by her father there led to her own presumed contract with the Spirit. She possesses long, curly blonde hair typically tied with a large red ribbon, blue eyes, and stands 166 cm tall. Her academy uniform features distinctive stocking-like socks and black shoes.

Proud of her noble lineage and family name, she reacts negatively to nicknames like "Montmo". While generally mature and amiable with classmates, her relationship with on-again, off-again boyfriend Guiche de Gramont is marked by volatility due to his infidelity, triggering intense jealousy and confrontations. She exposed his two-timing early on, publicly humiliating him, and though emotionally tied to him, her trust diminishes despite intermittent reconciliations signaled by gifts like a Mythril brooch.

A water mage classified as Line (Dot), Montmorency demonstrates skill in potion-making, notably creating a prohibited love potion intended for Guiche. Its accidental consumption by Louise de La Vallière forced Montmorency to seek an antidote. She orchestrated a journey to Lake Lagdorian, using her blood via her orange frog familiar Robin to contact the Water Spirit. The Spirit, angered by Cromwell's theft of its Ring of Andvari, agreed to provide a Spirit Tear after Saito Hiraga promised to retrieve the ring. In another incident, accidental exposure to a different love potion caused a brief infatuation with Julio Cesare, though her memory was later erased.

She participates in academy activities like the Familiar Exhibition with Robin and collaborates with peers during crises, such as Fouquet's attacks. Her dynamic with Kirche von Zerbst evolves into a more cooperative relationship, particularly during the resolution of the Water Spirit crisis and other shared challenges.