Introducing Tokyo revengers

Tokyo Revengers:

The plot follows Takemichi hanagaki, a boy who at 26 years old thinks he has already ruined his life. Living on odd jobs good enough to pay the rent, his life gets even worse when he learns of the death of Hinata, the only girlfriend he's ever had; just when he thought things couldn't get any worse, he discovers that Hinata Tachibana, his ex-girlfriend, has been murdered by the Tokyo Manji Gang: a group of dreaded criminals who have been threatening the peace of society for some time.

Still in shock over Hinata's death, Takemichi is the victim of an accident that takes him back 12 years, when he was in high school. 

Hanagaki begins to wonder when things went wrong, until he suddenly travels back in time 12 years ago - when he was still in a relationship with Tachibana. Hanagaki realizes that he now has a chance to save her, and to do so he infiltrates the Tokyo Manji Gang, aiming to climb the ranks and manage to change the future to save Tachibana from her tragic outcome."

As time goes by, he is reunited with his old friends but he is affected by the fact that "Kiyomasa", a senior student has him as a slave. 

The outlook starts to cloud as everything happens very fast and he is already lying on the floor beaten, about to give up, when he thinks of visiting "Hina" who is still his girlfriend at that time.

(12 years in the past) which gives him a push in his heart to save her.

At one point he finds some bullies picking on a kid which makes his blood boil and he beats them up until they decide to run away from him.

He gets a big surprise as the bullied kid is "Naoto Tachibana" yes, his girlfriend's brother! 

He decides to tell him everything as not only his sister was killed 12 years in the future but Naoto (brother of Hina, takemichi's girlfriend) was also killed, in the state of shock he decides to
believe him and they shake hands, which makes Takemichi feel a great shock in his body and in the blink of an eye he finds himself back in the present 12 years in the future, THAT WAS IT! when he shakes hands with Naoto he can travel 12 years in the past, that is from 2017 to 2005.