Elias Ainsworth possesses an enigmatic origin, blending fae and human aspects with an unclear true nature. His earliest recollection is awakening in a forest and wandering until collapsing before Lindel. Lindel brought him to Rahab, who bestowed the name "Elias Ainsworth" and urged Lindel to train him in magic. Lindel became his mentor; they led a nomadic existence herding caribou before eventually parting ways. Elias then settled in Britain, undertaking tasks for the Church.

Typically, Elias manifests as a tall humanoid with purple skin. His head is an animal skull resembling a wolf with goat horns, featuring glowing red lights in the eye sockets. He wears a gold-stringed necklace connected to a red cloth veil, often covering his face outdoors, and dons a tailored suit. His monstrous true form is thinner and taller, displaying a feathered collar, claws, a tail, protruding thorns, additional curved horns, and a tattered cloak fused to his body. He can shapeshift into human forms like Simon or Chise, or into animals, though this demands concentration and falters under emotional agitation. He can also separate into two entities, splitting his consciousness and diminishing his powers.

Magically, Elias commands shadow manipulation, hiding within or traveling through shadows, and wields thorn-based spells, earning titles such as "Child of Thorns" or "Pilum Murialis." He also employs teleportation, memory alteration, and destructive magic. His reclusive nature stems from an inability to relate to humans or fae, both viewing him as an abomination. He studies human behavior intellectually but grapples with experiencing emotions firsthand, often describing them physically (like "coldness" for loneliness). This emotional deficiency leads to childlike tantrums and secrecy about his past, though he seeks amends for misdeeds through church-sanctioned tasks.

His journey centers on his bond with Chise Hatori, whom he acquires at an auction to serve as apprentice and future bride. Initially, he values her both as a rare "Sleigh Beggy" whose magic might extend her foreshortened lifespan and as a means to grasp human emotions. Protective instincts towards Chise sometimes unleash uncontrollable rage, such as attacking Joseph for harming her. Yet, his lack of emotional insight causes severe ethical lapses, notably attempting to sacrifice Stella Barklem to transfer Chise’s dragon curse—an act driven by jealousy over their friendship. This caused Chise to leave him temporarily, forcing Elias to confront his flaws.

Gradual exposure to Chise fosters emotional growth. He experiences loneliness during her absence and begins respecting her autonomy, supporting her investigation into college threats despite risks. Their relationship evolves into mutual dependence, culminating in an informal exchange of rings symbolizing commitment. Later, he becomes a professor at the magical college, using his knowledge to guide students while navigating his burgeoning humanity.