Discovered amnesiac by a human boy named Gabriel, the character known as "Mysterious Boy" or "Evan" appears in the OAD *The Ancient Magus' Bride: The Boy from the West and the Knight of the Blue Storm*. In this "Lost Form," he resembles a prepubescent boy with fair skin, shoulder-length white hair, and red eyes. He wears a blue tunic over a pale undershirt, grey pants, a brown belt, red cloth shoes, and a green short sword in a pale scabbard at his belt. A Master of the Hunt Hunting Horn around his neck signifies his role as leader of the Wild Hunt.

His amnesia lends him a melancholic demeanor. Over two days, a strong friendship forms with Gabriel, offering him his first experience of the human world outside the Wild Hunt. This friendship proves pivotal. When Gabriel wishes to join the Hunt, he agrees and takes him onto his horse. However, when Gabriel changes his mind, his inherent fae possessiveness in matters of love and friendship causes initial resistance. Despite this nature, he ultimately respects Gabriel's choice. While restrained by Elias Ainsworth, he assists the fearful Gabriel in dismounting.

Regaining his memories returns him to his "True Form." His fair skin turns deathly pale, his red eyes fade, and his white hair lengthens to his waist, now braided in several places, with blue markings appearing on his face. He dons armor and a red cape, reflecting his restored status. His personality shifts to greater confidence and a noble bearing consistent with his role as the Knight of the Wild Hunt. His restored identity includes a close bond with a black horse, his steed and companion. Their reunion features mutual affection as they gently hold each other close.