Leo was hatched in the rookery of Knight's Spur in 1898.  With his rookery siblings, he was probably taught the Excalibur Poem by his elders.  At some point before 1940, he was one of the gargoyles who volunteered or was chosen to work at Into the Mystic, the clan's shop in Soho, along with his rookery siblings Griff and Una.
At the beginning of World War II, during the Battle of Britain, Griff felt it his duty to help defend London from the Nazis, but Leo and Una viewed the war as a "human problem", and chose to simply continue running the shop and let the humans fight the war. When Goliath arrived in 1940 via the Phoenix Gate, he chose to join with Griff, and accompanied him on his last engagement in the Battle of Britain, before both vanished (due to Goliath's taking Griff forward in time to 1996).
After Griff and Goliath never returned home to the shop, Leo and Una believed that they had been killed in the fighting. In their mourning, they helped to finance and build a memorial to the fallen heroes of the Battle of Britain near the Thames River.  Over the next fifty-six years, Leo and the rest of the clan slowly lost the Gargoyle Way. They continued to view the shop and their rural estate as their protectorate, but they turned a blind eye to the street crime in their neighborhood, which they viewed as somebody else's problem. Linked by their grief over Griff's loss, Leo and Una grew closer together and became mates.  They were rookery parents to the hatchlings of the 1958 and 1978 rookeries. Indeed, Lunette is their biological daughter.  They were of the correct age to parent the 1998 rookery as well, but probably did not do so due to the London Clan's population limiting system.
When Goliath and his companions arrived in London in early 1996, Leo and Una were shocked to see he had survived and they blamed him for the assumed death of Griff - partly because of their buried guilt. They knew, deep down inside, that they should have helped protect London alongside Griff. Leo and Una angrily accused him of being responsible for Griff, and sought revenge by capturing Elisa, Angela, and Bronx and chaining them up in the shop's basement. Goliath realized that the only way that he could save his friends was by going back in time to 1940 to discover what had happened to Griff, and used the Phoenix Gate to return there.
After his departure, Leo realized, and convinced Una of this as well, that the real reason for their anger was their buried guilt, and freed Goliath's companions. When Goliath returned with Griff shortly thereafter, Leo and Una forgave him, and afterwards expanded their protectorate to cover London rather than just the shop and the clan's estate, protecting the innocent from those who would prey upon them. Leo, who had spent most of his life only focused on the business of the magic store, now made the well-being of the community a priority. ("M.I.A.")
Leo looks very much like a winged bipedal lion; hence his name. He has rounded ears, a full mane, and a long tail ending in a tuft of fur. He has feathered wings and his skin coloring is brown. His hair is a tannish-gold, and by 1996, two streaks of gray had developed over his forehead. He wears a sort of light armor and, occasionally, a long dark robe.
Behind the Scenes
Voice Actor: Gregg Berger