In the 4th OVA episode in Season 3, Yumi's father asks her what he should wear to the Sports Carnival at Lillian. Yumi's mother then comments that he has a sort of rivalry with Sachiko's father. When Yumi tells her father that Sachiko's parents probably won't be coming because of a wedding ceremony they are attending, he seems disappointed.
How did Yumi's father know Sachiko's father?
