Very little is known of Leo's pre-plague life. At the time of the superflu outbreak he is eleven years old and living in Epsom, New Hampshire. Nadine Cross finds Leo dying on the lawn of his house from an infected animal-bite; inside the house, his parents and three siblings are dead of the plague.
Nadine nurses Leo back to health, but finds that he has reverted to a feral state: violent, refusing to wear clothes, unable to speak or even tell Nadine his name. She calls him "Joe" and he fiercely bonds with her; but it takes all of Nadine's experience and skill from working with children to keep his murderous impulses in check.
Nadine and "Joe" encounter Larry Underwood in Maine and begin to follow him. Nadine wants to travel with Larry, but does not trust Leo, who has taken to wielding a large knife, to not hurt him. Finally they do make contact and as expected, relations between Leo and Larry are tense and uneasy at first.
There is a breakthrough when the pair discover that Leo appears to be a musical prodigy with a photographic memory. One night on the beach Larry is playing an expensive guitar he found; Leo is entranced, and eventually gestures for him to hand over the instrument, whereupon he demonstrates the ability to play every one of Larry's song verbatim (although clumsily, indicating he has never actually held a guitar before.)
Leo tentatively begins to bond with Larry and even begins to speak again, in monosyllables.
After Lucy Swann joins them, it is through Leo that they realize they have all been sharing the same nightmares about Randall Flagg, as well as the same dreams of Hemingford Home, Nebraska and Mother Abagail. The group heads towards Nebraska, then to the Boulder Free Zone.
Upon arriving in Boulder Larry's group goes to greet Mother Abagail. There is a tense standoff between Nadine and Abagail, broken when Nadine puts Leo between them as a human shield. At that moment, Leo's full powers of speech return. He is finally able to speak in complete sentences, and tells Abagail his real name, running to embrace her.
In Boulder Leo begins to display a strong psychic ability — possibly the explanation for his latent musical talent, as he can read Larry's mind with exceptional clarity. He is afraid of Harold Lauder because he perceives his true nature, theorizing that behind Harold's smile are "big white worms eating up his brain like maggots" and even wondering if he prays to Flagg behind closed doors. Leo will not even set foot on Harold's property let alone enter his house.
"Nadine-mom" and Leo become estranged as she falls more and more under Flagg's influence. When Nadine leaves her home to cohabit with Harold, Leo briefly relapses into his feral "Joe" state; for her part Nadine feels betrayed by Leo, as he chose alliegance with Mother Abigail over his love for her. At this point Leo goes to live permanently with "Lucy-mom" and Larry.
It is Leo who warns Larry that Harold is up to no good in Boulder, and that the details are "all written down", urging him to join forces with Fran Goldsmith. Piecing together Leo's clues, they surmise that Harold has been keeping a diary, which they find after breaking into his house. But they are unable to deduce the specifics of Harold's plan until too late.
From that point onward Leo is seldom mentioned in the novel and mostly drops out of the narrative. He is mentioned as still living with Lucy in Boulder the spring after the plague.