Grandpa Shinshuu Makabe is Masamune's grandfather, and he helps the boy shape up.
Grandpa Makabe was very strict when training Masamune to slim his body down into a skinny, handsome boy. He forced Masamune to exercise and do labor while dieting at the same time.
Grandpa Makabe is always wearing a traditional yukata, with a thin overcoat. He also carries a cane whenever he walks.
Not much is revealed about Grandpa Makabe other than that he seems to be strict and angry towards Masamune whenever it involves keeing his grandson handsome and slim. He orders Masamune around to listen to him, promising that the effort will be worth it towards becoming handsome.
8 years prior to the story, Masamune was nicknamed "Piggy." This caused a lot of bullying to occur, and Masamune grew tired of it all. He wanted to change so people would no longer call him that. Changing his surname from Hayase to Makabe, Masamune then asked his grandfather to train him to become fit and handsome.
Masamune went through a lot of pain and effort, changing his whole lifestyle to what Grandpa Makabe ordered. This resulted in his current form, satisfying both Masamune and Grandfather. With the results he wanted, Masamune left his Grandfather and returned to society as his new self.
In Chapter 47, it is shown that Masamune left his family and returned to his grandfather to have some time by himself and think about his current situation. Grandfather is still just as strict, admonishing his grandson for bringing candy to his place.