The Little Brother Is The Academy's Hotshot summary:
In the last battle with the king of the dragon race that killed countless humans, Rudd Denatos loses his comrades and biological elder brother, Aesir Denatos. The moment he slays an unidentified dragon that appears before him, he returns to his days as an academy cadet. However, his brother, Aesir, a genius warrior doesn’t exist in the past world Rudd has regressed into. Trying to fill in the empty spot of Aesir, Rudd tries to prepare as much as possible before the strongest dragon king, the Seventh Dragon, appears. Will Rudd be able to find his older brother and avenge his parents?