Tower of the Future

Matsuyuki Takeru could not have anticipated the decidedly different turn his life would take. After his mother's death, life continued at a reasonably normal pace — he studied hard for exams, played fantasy role-playing games, even fell in love. But things began to change when he discovered a half-sister he didn't know about. A strange boy started appearing, claiming to be a warrior reborn who must fight an epic battle against forces of evil! When fantasy becomes reality, will every choice he makes affect the outcome?

source: mangaupdates.com/series/gcwbdo1/mirai-no-utena