Tenseisaki ga Shoujo Manga no Shirobuta Reijou datta reBoooot!

Synopsis:

When my grandfather informed me that my engagement had been called off, memories of my past life suddenly came rushing back. At the same time, I realized this world was identical to the setting of my favorite shoujo manga. The villainess who went to every extreme to torment the heroine… I had been reincarnated as her chubby lackey. Remembering the fate that awaited her made me tremble—once the villainess’ schemes were exposed, she would dump all the blame onto Britney. That was my role, and it ended in execution. To survive, I absolutely cannot remain the villainess’ follower. Since she only surrounds herself with girls she considers far less attractive than herself, there might still be a way out if I lose weight. For now, the plan is simple: time to go on a diet.