Leo is the heir of the Harwey family that owns the sixty of the real world.
Believing the Holy Grail to be potentially dangerous to humanity, Leo participates in the Holy Grail War to secure it. Confident in his abilities, he refers to his Servant Saber by his true name and does not make him hide his form around others. He doesn't concern himself with subtlety or tactics, instead letting something destined to be revealed happen without question. Victory is all but assured to him, as his superiority over others has been reinforced every day since he was born. Though he was raised to be a perfect ruler, he lacks normal human emotions, feeling despair for the first time only when he is defeated.
He is able to enthrall and enchant the entire class during the preliminaries with his regal bearing. He naturally seems better than those around him, as if he is naturally looking down upon them in his place as king. Like a prince, he has a natural charisma that draws people up to his level. His smile brings about the same sort of feeling as a warm and comforting sun rise, and his presence is inviting and inclusive. Any worries vanish and others begin to think of doing as he says, as if it is the only way.
Despite his overall kind disposition, just looking upon him brings about the Initial Protagonist's fight or flight instincts. Rather than kindness, he senses something malicious behind his appearance.
Leo is introduced during the preliminaries, where he proclaims that his name will be known to all the world in time. He is not affected by the brainwashing effect, so he mostly takes his time to scout potential opponents like Rin Tohsaka and casually enjoy his school life, such as flirting with Taiga Fujimura by stating he would like her to visit his country.
His introduction begins to shift the state of the world and starts to tear down its repetitive nature. As he is the distinguished son of the Harway plutocratic family, in a sense the person who will eventually rule the world, his presence in the school is strange to others. He takes Shinji's female followers and, unlike Shinji, helps them without showing any condescension. When Shinji begins to chide him, he apologizes with dignity for upsetting him without any hostility or resentment.
Towards the end of the preliminaries, he flirts with Taiga again before leaving, calling her a remarkable person that reminds him of the beauty of youth. He mentions that found the school to be an interesting and enjoyable experience, as he had never been able to attend one.
While Magus in the Fate/Extra universe do not possess the same abilities of their standard counterparts, Leo is noted for possessing several unnatural traits. Firstly, he possesses an other worldly charisma, an air of superiority that causes others to gravitate towards him. This is likely a manifestation of the type of "Charisma" that great kings and leaders are known to have, especially amongst heroic spirits.
While the extent of which is unclear, Julius' flashbacks detail that Leo went through repeated surgery as a child, including a thaumaturgical process to burn knowledge directly into his brain. This likely included other processes in order to give him traits befitting the Harwey heir.