Originally introduced as a human member of Fairy Tail, Loki (also known as Loke) consistently wears light-tinted sunglasses and exhibits a highly flirtatious personality, frequently surrounded by multiple girlfriends. This behavior earned him a top ranking in the "Mage you want to have as a boyfriend" category within Sorcerer Magazine. Despite his playboy demeanor, he demonstrates strong protective instincts toward women and unwavering loyalty to his friends. He displays an early aversion to Celestial Spirit Mages, particularly Lucy Heartfilia, hinting at a deeper history.

His true identity is Leo the Lion, leader of the twelve Golden Zodiac Celestial Spirits. Before joining Fairy Tail, he was contracted to Karen Lilica of Blue Pegasus. Karen treated her spirits, especially the timid Aries, with severe cruelty—using Aries as a shield and forcing her into uncomfortable situations. Witnessing Karen threaten to punish Aries with seven days confined in the human world—a potentially fatal act—Leo intervened. He forcibly took Aries's place and issued an ultimatum: Karen must terminate her contracts with both him and Aries, or he would remain in the human world indefinitely, blocking her from summoning other spirits. He resisted her attempts to forcibly close his gate using his own magic.

Remaining in the human world for three months despite the gradual drain on his life force, Leo eventually decided to forgive Karen, only to discover she had died attempting a mission without her spirits, as she could only summon one at a time while he was active. Blaming himself for her death, he was exiled from the Celestial Spirit World for violating his contract and causing his master's demise. Adopting the alias "Loki," he joined Fairy Tail as a human wizard, concealing his true nature, though his non-human essence was sensed by Bickslow. His extended stay, totaling three years, was exceptional, as non-combat spirits typically fade after seven days.

As Loki began fading due to his prolonged time outside the spirit world, Lucy Heartfilia investigated his strange behavior and uncovered his secret. Confronting him at Karen’s grave, Loki revealed his past. Lucy refused to let him die and attempted to force open his gate, nearly merging her life force with his. This act summoned the Celestial Spirit King. Despite Loki’s exile, Lucy argued passionately for his life, demonstrating extraordinary resolve by summoning all her Celestial Spirits simultaneously—a feat demanding immense magic power that caused her to collapse. Witnessing her determination, the Celestial Spirit King revoked Loki’s exile, allowing him to return as Leo. Loki, moved to tears, expressed gratitude to Lucy and formed a contract with her, giving her his key.

After his return, Leo became one of Lucy’s most trusted spirits, declaring himself her "knight in shining armor" and shifting his flirtatious focus primarily toward her, though he maintained friendly relationships within the guild. He retained his signature Regulus magic, a light-based power activated via a magic ring worn on his finger. This magic enhances his physical attacks with destructive light, often manifesting as lion-like energy projections. Key techniques include Lion Brilliance, which emits blinding or offensive light over a wide area, and Regulus Impact, a powerful punch that materializes a lion’s head to blast opponents away. He is also a master hand-to-hand combatant, capable of incapacitating foes swiftly even without magic. His durability and resilience are notable, stemming from his role as a combat-oriented spirit.

Throughout Lucy’s adventures, Leo consistently demonstrates loyalty. In confrontations such as the battle against the Garou Knights, he retrieves stolen Celestial Spirit keys, ensuring Lucy and Yukino Agria can fight effectively. He actively participates in battles, coordinating strategies with other spirits like Virgo, Taurus, and Libra to overcome adversaries such as Uosuke. His protective nature toward Lucy remains a defining trait.