Leonardo B. Harwey, also known as Leonardo Bistario Harwey, is the heir to the Harwey family, which controls sixty percent of the real world's wealth. He participates as a Master in the Moon Cell's Holy Grail War, commanding the Servant Saber, whose true name is Gawain. Born into immense privilege, Leo was identified as a prodigy at the age of three and subjected to rigorous training and surgeries to enhance his abilities, including a thaumaturgical process that directly imprinted knowledge into his brain. This upbringing was designed to shape him into a perfect ruler, but it also left him largely devoid of normal human emotions.

Leo is calm, polite, and confident to the point of appearing detached. He does not rely on deception or subtlety, preferring to let events unfold as they will, secure in the knowledge that his superiority makes victory all but assured. His motivation for entering the Holy Grail War is not personal ambition but a deep-seated belief that the Holy Grail poses a serious threat to humanity. He seeks to secure it in order to stabilize the world and guarantee humanity's survival for the next millennium. He prioritizes the continued existence of mankind over the rule of his own family, viewing the Harwey family as a means to that end.

In the story of Fate/Extra Last Encore, Leo appears as one of the most formidable Masters, having won the previous Holy Grail War. He acts as a key obstacle to the protagonist Hakuno Kishinami and his Saber. Leo's role is that of an antagonist whose ideals contrast sharply with Hakuno's. While Hakuno represents change and struggle, Leo represents order and stagnation. His relationship with his Servant Gawain is built on mutual respect and a shared understanding of kingship. Leo also has a complex history with his half-brother Julius B. Harwey, who was deemed unqualified to lead their family.

Throughout the series, Leo shows little personal development, as his acceptance of his role and his lack of self-interest leave him unchanging. However, when faced with defeat, he gains a glimpse of a more human perspective. His abilities as a Master are extraordinary; he is described as the strongest Master in the history of the Holy Grail War, with all his parameters at maximum. He is also capable of advanced magic, such as terrain manipulation on a massive scale, and possesses great computational power and strategic acumen. His servant Gawain, the Knight of the Sun, is a formidable Saber-class servant whose strength is amplified in sunlight. Together, they form an almost unbeatable combination. Leo's ultimate fate in Last Encore is tied to the rejection of his vision for a static, peaceful future, as the narrative suggests that such a path would lead to humanity's eventual stagnation and extinction.