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Sakai Yuji is a normal boy living his new highschool life in Misaki High. His world turns upside down when he met the Enpatsu Shakugan no Uchite or the Flame-Haired Blazing-Eyed Hunter and learned that the real Sakai Yuji is already dead and he is only a torch (a temporary replacement of the people who’s “Power of Existence” has been eaten by a tomogara). Fortunately, he is a mystes (a torch with a hidden treasure inside it) and that the treasure inside him is a Reiji Maigo. Unlike other torches he can replenish his power of existence every midnight.

Yuji is a kind person who puts others before himself. This often draws much confusion and annoyance from Shana, as prior torches from Shana's experience exhibited selfish attitudes. Simultaneously, Yuji's compassion and empathy sow the seeds of attraction between him and Shana.

Upon Chigusa's pregnancy, it was revealed by Kantaro that Yuji actually had a twin brother who died at birth. Thus, in memory of the first baby and from the parents' wish, the second baby was given the name Yuji, which means "Second (child) who shall live long ".

With the Reiji Maigo, Yuji has the possibility to live on like an average human as long as he makes sure that his Power of Existence does not run out before midnight. Due to Reiji Maigo, no one is sure if Yuji will age like a regular human or be immortal as he is. Since Reiji Maigo restores his power of existence, Yuji is training to use that power in the hopes of fighting alongside Shana. In the second season of the anime, it is mentioned than Yuji's power of existence can rival that of a Lord of the Crimson Realm, in terms of quantity.

Even when Yuji was ignorant of the fact that he possessed the Reiji Maigo in the first episodes of the first season of Shakugan no Shana, Yuji develops a somewhat optimistic, yet ultimately fatalistic view of his life. Even when he believed he was slated to perish as a Torch, Yuji took the information in a stride after the initial shock, and did not seem too disturbed or melancholy with the idea that everyone would simply forget about him, accepting this. Rather, his acts of heroism, demonstrated often in the series, is justified by his belief that even if his existence would one day disappear, his sacrifice would be proof that he had once lived.

Although Yuji lacks of the training and discipline of a Flame Haze, he has demonstrated an admirable amount of intelligence and logic, often thinking outside the box and solving puzzles others can't (though it still stands for a fact that he is totally clueless to the fact that Shana loves him and is often jealous of any girls who get too close to him). While many Flame Hazes and Lords of the Crimson Realm would be deceived or confused by unconventional casting of magic, Yuji is able to quickly deduce and see through the ploy despite his unfamiliarity with many magical spells, enabling his allies, generally Shana, to strike at the heart of their opposition. Furthermore, by the start of the second season, Yuji's training has enabled him to foresee as well as dodge a good majority of attacks from skilled Flame Hazes such as Shana and Wilhelmina, and he has gained the ability to cast Power of Unrestraint. He also has the ability to wield Blutsauger and infusing it with some of his own power. He also can create his own fire, which is silver in color, to fire at targets.

Source:
http://en.wikipedia.org/wiki/List_of_Shakugan_no_Shana_characters
http://www.anime-source.com/banzai/modules.php?name=Content&pa=showpage&pid=1063
http://www.crunchyroll.com/group/Shakugan_no_Shana_loverz