MASAMUNE-KUN'S REVENGE

As an overweight child, Makabe Masamune was mercilessly teased and bullied by one particular girl, Adagaki Aki. Determined to one day exact his revenge upon her, Makabe begins a rigorous regimen of self-improvement and personal transformation. Years later, Masamune re-emerges as a new man. Handsome, popular, with perfect grades and good at sports, Masamune-kun transfers to Aki's school, and is unrecognizable to her. Now, Masamune-kun is ready to confront the girl who bullied him so many years ago and humiliate her at last. But will revenge be as sweet as he thought? (Source: Seven Seas Entertainment) — Description from AniList community (anilist.co).