Loki, also known by his true name Leo, is the Celestial Spirit of the Lion, one of the twelve Golden Zodiac Keys, and serves as the leader among them. He first appears as a flirtatious and charismatic young man who has joined the Fairy Tail guild, where he quickly becomes popular, especially among female members. However, his outward charm masks a deep inner conflict rooted in his past. Originally belonging to the Celestial Spirit mage Karen Lilica of the Blue Pegasus guild, Leo witnessed her abusive treatment of another spirit, Aries. Unable to bear it, he forcibly remained in the human world to protect Aries and pressure Karen into releasing their contracts. This act inadvertently led to Karen's death, as she could not summon other spirits while Leo was present and was killed on a mission. Leo was then exiled from the Celestial Spirit World, blamed for her death, and forced to wander the human world under the alias Loki. Over the following three years, his prolonged stay in the human world gradually drained his life force, bringing him close to death.

Driven by guilt and a desire for atonement, Loki initially keeps his true nature hidden, but his path changes when he meets Lucy Heartfilia, a fellow Celestial Spirit mage. He grows close to Lucy and eventually reveals his secret. With Lucy’s unwavering belief in him and her plea to the Celestial Spirit King, his exile is revoked, and he becomes one of Lucy’s contracted spirits. From that point, he declares himself Lucy’s knight in shining armor and becomes a fiercely loyal and protective ally. His relationship with Lucy evolves from playful flirtation into a deep, abiding friendship built on trust. He also shares a strong bond with Aries, whom he continues to watch over with care, and has friendly connections with other Fairy Tail members such as Natsu and Happy.

In terms of abilities, Leo wields the magic known as Regulus, which manifests as radiant, lion‑themed attacks. He can fire beams of light, summon a giant flaming lion, and perform powerful physical strikes enhanced by his magic. He is also a skilled hand‑to‑hand combatant, often using a magical ring as a weapon. In an alternate timeline known as the Eclipse Celestial Spirits arc, which appears only in the anime, Leo becomes the Eclipse version of himself. In this form, he abandons his usual suit and sunglasses for black armor, grows a long mane‑like hair, and gains the corrupted magic Dark Regulus, which absorbs light and manifests as dark flames. This dark version is consumed by a desire for perfect freedom and rebels against his former master, though even then he retains faint memories of Lucy.

Throughout the story, Leo undergoes significant development. He moves from a place of guilt, isolation, and self‑destructive acceptance of his impending death to a renewed sense of purpose and belonging. His exile and redemption arc emphasize themes of forgiveness, friendship, and the value of life. After being cured and returning to the Celestial Spirit World to recover, he later returns to Lucy’s side, now healthier and more vibrant, yet still carrying the weight of his past as a source of strength. He remains a steadfast companion, one of Lucy’s most powerful spirits, and a beloved figure within the Fairy Tail guild.