Anomaly System Quadrangle

In high school, Maki Eiji was a delinquent who spent more time skipping class and getting into fights than studying. During these turbulent years, one person stood out - Chino Kotoha, a kind girl who showed him compassion despite his reputation. However, their innocent connection had unintended consequences that would haunt Eiji for years to come. Now an adult, Eiji unexpectedly encounters Kotoha again, and the weight of his past actions crashes down on him. When a mysterious woman appears with an unbelievable proposition - the chance to return to his high school days and potentially change what happened - Eiji faces an impossible choice. But altering the past proves more complicated than simply righting old wrongs, as he discovers that the events of those days were more complex than he ever realized. (Source: MangaHelpers)