Attack on Titan: No Regrets(2014)

Attack on Titan: No Regrets

This prequel to megahit Attack on Titan answers the questions: How did Captain Levi of the Survey Corps go from street thug to humanity's strongest soldier? And how did Commander Erwin become a cold, calculating leader, ready to sacrifice anything to save the human race? The fires that forged this bond of loyalty and trust were intense indeed!

(Source: Anime News Network, revised)