In the Manga
Leon is a young boy with a great distrust of adults and society, as he is a survivor of a war where all adults were killed, including his parents, and only children surived. He travels on a snowboard, and has the power to control air, managing to suffocate people, control air currents, among many things. Leon's Tao Power is called "Wind".

In the Anime
He is part of a group of orphans that includes Tim Vertical. Constantly being ignored or abused by the "adults" of society, Leon sought more power so that he could better protect his friends. Ironically, after he did gain this power, he had to leave his friends behind so that he could help the Apostles remake the world. Near the final part of the story, Leon's the one who volunteers to get the fireworks for Eve to hear to trigger a reaction that would stop her from uniting with Eden. He is saved by Train when he is at the limit of his strength and is about to be taken down by two monsters from Eden.

Source: Wikipedia