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Menace

The manga centers on Teruya, a delinquent teenage boy who spends his days fighting as part of a gang, but decides to quit his gang for the sake of his beloved Aimi. As part of his retirement from the gang, he undergoes a "retirement ritual" where the gang beats him up badly. After the beating, he wakes up and finds that he has lost his memories, even the knowledge of his name. It is then that he meets a strange alien creature.

(Source: ANN)