Hidaka Haru manga
From Hoshikuzuu Scans: The story of a young owner of a long-standing small eatery and a puppy-like laborer. Kota attaches himself like a puppy to Makoto, who has been like a fatherly figure to him. Even after they have grown up, he still smiles and proclaims proudly that “Mako-san’s food tastes just like a mother’s does!” The two get along as well as a family, but one day, a customer arrives who becomes a thorn in Kota’s side. The two appear friendly and Makoto shows expressions that Kota has never seen him make. And he acts differently than normal… Kota is then plagued with a dislike and worry that he can’t shake off.