Leo was a member of the London Clan of gargoyles, who operated the magic shop In To The Mystic that provides the clan's main source of income. Leo was a friend of Griff in 1940, but when he disappeared for over 50 years, he later became the mate of Una, who had been attracted to Griff back then. He and Una both became embittered, professing to blame Goliath (who had time-travelled back to 1940 and disappeared back to the present with Griff) for Griff's death.
After Goliath came to the shop in 1995, Leo and Una tried to get revenge by harming Goliath's friends, but Leo later realized that what they were actually feeling was guilt, guilt for not going with their friend. At that realization, he let the prisoners go and they forgave Goliath- just as soon as he returned with a confused Griff. They have since continued to mind the shop, being happy to have their old friend back.