The Extra's Ascension.

After dying alone in his apartment, guilt-ridden orphan Aelric wakes in the body of Aldric Ashford—a sixteen-year-old extra from the game "Eclipsed Heart" who was killed by bandits and left for dead in the Forest of Echoes. Through fragmented memories, he discovers the original Aldric had a younger sister, Sera, who lies in a coma with mana core degeneration requiring fifty thousand gold for treatment, money they don't have. With only three months until Lumina Academy's entrance exam, Aldric must survive the forest, reach the city of Veldra, and transform his dead extra's body into something capable of passing the exam, entering the academy, and finding a way to save the sister who's now his only reason to keep going.