Rikka goes to the same school as Yuuta. Throughout the series they make the "contract of lovers".
Rikka is Sanae's Master and best friend. They met on the internet two years prior to the start of the story. They have a very close relationship and share their interest in chuunibyou behaviour, often acting battles with eachother and casting "magic spells". Sanae looks up to Rikka and cares for her a lot, becoming upset when she threw away her true desires and agreeing to be forced to move with her grandparents. She became just a little jealous when Rikka began dating Yuuta, but eventually got over it and gave them her support.