Summer Wars Part 1 (manga)

When timid eleventh-grader and math genius Kenji Koiso is asked by older student and secret crush Natsuki to come with her to her family’s Nagano home for a summer job, he agrees without hesitation. Natsuki’s family, the Jinnouchi clan, dates back to the Muromachi era, and they’ve all come together to celebrate the 90th birthday of the spunky matriarch of the family, Sakae. That’s when Kenji discovers his “summer job” is to pretend to be Natsuki’s fiancé and dance with her at the birthday celebration.