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.
(Source: High School DxD Wikia)