Shakugan no Shana

Yuji Sakai, a high school student, is told by strange girl that he died some time ago, and that he is a "Torch", a temporary replacement for erased humans. Torches take on the forms of those erased persons, but after a period of time, they will vanish from existence and the memories of the living. Unfazed by his apparent death, Yuji befriends this girl and names her "Shana" after her sword. Shana resolves to protect Yuji from the Denizens, who would use his special replenishing Torch to disrupt the balance of the world, and Yuji decides to join Shana in her fight.

(Source: Anime News Network)

Note: Chapter count includes two "extra" chapters.