Leonardo Bistario Harway, heir to the Harwey family controlling sixty percent of the world's wealth, is the biological son of the former Harwey head and Alicia. His older half-brother, Julius B. Harwey, was deemed unqualified to lead. Identified as a prodigy at three, Leo underwent repeated surgical and thaumaturgical procedures to implant knowledge directly into his brain, distancing him from Alicia. Julius received orders to assassinate Alicia to secure Leo's inheritance, an act carried out per family directives.

Leo possesses a charismatic and regal demeanor that naturally enthralls others, often described as radiating warmth like a sunrise, compelling obedience and dispelling worries. Despite this outwardly kind disposition, his aura triggers instinctive fight-or-flight responses in protagonists. He interacts coldly with artificial intelligences like Sakura Matou and Rani VIII, denying them honorifics while acknowledging their identities. Raised as a "thorough ideal" ruler, Leo embodies stagnation and defense, contrasting protagonists representing change and offense. He lacks normal human emotions, experiencing despair only upon his first defeat, and cannot comprehend failure due to his conditioned superiority.

Believing the Holy Grail could ignite global conflict if attained individually, Leo participates in the Moon Holy Grail War to secure it for stability rather than power. His objective prioritizes humanity's continuation over Harwey rule, aiming to establish a millennium of safety. He considers those opposing his family's governance expendable obstacles to global equilibrium.

In Fate/EXTRA, Leo serves as Master to Saber (Gawain), openly revealing his Servant's true name without tactical concealment. He scouts opponents during preliminary school phases, demonstrating fascination with ordinary experiences denied during his upbringing. He confronts Hakuno Kishinami, framing their conflict as ideological opposition between stagnation and change. After Hakuno defeats Julius, Leo acknowledges them as a worthy adversary while callously accepting his brother's death as foundational to his rule.

During Fate/EXTRA CCC, Leo falls into the Far Side alongside other Masters. Though his memories are sealed, BB cannot brainwash him. Retaining Gawain, he leads the student council to escape. Facing BB in Chapter Bad End, he sacrifices himself by expending all Command Spells to break his contract with Gawain, preventing BB's absorption of the Servant. BB kills him immediately after, but not before he transfers student council leadership to Hakuno.

In Fate/Extra Last Encore's alternate timeline, Leo loses to Hakuno approximately nine hundred years before the main events. Rescued from death by Twice H. Pieceman, he embraces Twice's philosophy advocating the erasure of human history. Transformed into the "King of the Land," Leo oversees the stagnant Holy Grail War system from the Moon Cell's pinnacle, enforcing a twisted perpetuation of the conflict.

Physically, Leo appears as a 14-year-old student, wearing a custom orange Tsukumihara Academy uniform in the Near Side and standard black in the Far Side. His Last Encore incarnation manifests as an older avatar clad in a beige trench coat. His abilities include "Kingly Charisma" and specialized Codecasts like "Sword Camelot: Castle of the Assembled Holy Sword," which traps Saber-class Servants and their Masters in honor-dueling arenas. These derive from sound thaumaturgy implants rather than traditional magecraft training.