Tokyo Revengers

Watching the news, Takemichi Hanagaki learns that his girlfriend from way back in middle school, Hinata Tachibana, has died. The only girlfriend he ever had was just killed by a villainous group known as the Tokyo Manji Gang. He lives in a crappy apartment with thin walls, and his six-years-younger boss treats him like an idiot. At the height of his rock-bottom life, he suddenly time-leaps twelve years back to his middle school days. To save Hinata, and change the life he spent running away, hopeless part-timer Takemichi must aim for the top of Kanto's most sinister delinquent gang. <em class=de-emphasized>(from manga)</em>

Synopsis from Anime News Network