Tatara and Chinatsu are both in the same class. Tatara had the worst impression of her, because of Chinatsu's abrupt insult on his hobby, and called it "So lame". After that day, they never interacted with each other again.
When Sengoku came back to Japan and has a competition, Tatara and Gaju excitedly came to watch the Sengoku-pair at Korakuen Hall. After the competition ended, Tatara accidentally bumped into Chinatsu, and was shocked seeing her in a dance competition that she insulted. Ashamed by being seen by Tatara, Chinatsu starts walking away and tells Tatara "If you talk to me at school, I won't forgive you.", and walks away again.
After some certain events, Tatara discovered Chinatsu was actually a former dancer and they end up as a pair and begins competing in professional dancing matches. While their relationship is complicated, it is implied that they're starting to like and love each other's company despite quarreling a lot.