He is shown to be a short, dark-skinned young boy with gray-blue hair and purple eyes. Like other members of the Hero Faction, he wears a gakuran, with a long coat on top of it.
Leonardo is shown to be a very quiet boy and is always seen without any expression on his face.
Not much is known about Leonardo's past, other than that at some point before the series, he was persuaded by Cao Cao to join the Hero Faction.
He appears in Volume 9 along with the other Hero Faction members and created hundreds of antimonsters to fight against the Gremory group and Irina before retreating.
In Volume 11, he gets kidnapped by Shalba and is brainwashed into forcefully activating his Balance Breaker, creating thirteen gigantic monsters to attack the Underworld. This action, however, caused Leonardo to suffer severe physical and mental damage forcing him to be taken out of the front lines.
Leonardo was mentioned in Volume 16 by Cao Cao who claimed that Leonardo stayed behind with Georg who is researching about magic in the Realm of the Dead.
Annihilation Maker (魔獣創造, Anaiareishon Mēkā), Leonardo's Longinus, which is listed among the four strongest Longinus. It has the ability to create countless various monsters that the wielder can imagine from his shadow.
- Bandersnatch and Jabberwocky (破滅の覇獣鬼, Bandāsunatchi ando Jabawokkī): Leonardo's sub-species Balance Breaker of Annihilation Maker that creates 13 new antimonsters: 12 Bandersnatchs and one Jabberwocky . The Bandersnatch are over 100 meters tall and very powerful. They have the ability to create numerous smaller copies of themselves, where between 10 to 100 small monsters are created each time. The Jabberwocky is over 200 meters tall and has a humanoid shape and even more powerful that withstand attacks of "The Emperor", Diehauser Belial, and his group. Like the Bandesnatch, It can create numerous smaller copies of themselves. The Jabberwocky also has incredible regeneration powers.
- Leonardo was the youngest member of the Hero Faction.
- Leonardo was the youngest of the current Longinus wielders.