He was close with another Whitelighter, Natalie, who was in Whitelighter training around the same time he was. Ironically, the two were both Medics in World War II, while they were both humans. They quite possibly became Whitelighters at the same time. During the mid-late 20th century, Leo tried to hunt down and kill a powerful Darklighter called Rennek, who was killing several magic practitioners. After he apparently vanquished Rennek, The Elders rewarded him with the task of being the guardian angel of what would become three of the most powerful witches in history.