A young, shy and short boy also being a former member of the Hero Faction lead by Cao Cao. Leonardo was the former wielder of the Longinus, Annihilation Maker, the fourth strongest Longinus. This Longinus has the ability to create countless of powerful monsters, in variety of what ever the user can imagine forming from their shadow. During the Longinus's sub-species balance breaker known as "Bandersnatch and Jabberwocky" the user is able to summons 13 tremendously powerful monsters. 12 being known as "bandersnatch" these monsters reach heights of over 100m tall and have the ability to produce up to 100 smaller versions of themselves whenever the need to. The other monster produced during the balance breaker is known as the "jabbawocky" this monster is tremendous in power, it reaches a height of over 200m and holds a human-like appearance. The jabbawocky is so powerful it is able to withstand the most powerful rating game champion "the Emperor" Diehauser Belial and his peerage. Also alike the bandersnatch it also can create numerous smaller versions of its self. Another power of this monster is that it has unbelievable regeneration powers.

in volume 11, during the ending of the third story arc, Leonard is used and brainwashed by "Old Maou Faction" leader Shalba Beezelbub. To use his balance breaker to wreck havoc among the underworld. After this Leonardo suffered sever mental and physical damage being left unable to continue fighting in the front lines for the Hero's Faction. Also During volume 12 after the problem of the attack on the underworld was solved, Leonardo along with fellow Hero's Faction member Georg and Hero's Faction leader Cao Cao, were together sent to the realm of the dead had their Longinus sacred gears taken from then by the Hindish god Indra.

(Source: High School DxD Wikia)