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 each other 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 then agreeing to be forced to move in with her grandparents. She became a little jealous when Rikka began dating Yuuta, but eventually got over it and gave them her support.
Her older sister, whom she usually refers to her as "The Priestess" and is frequently seen to be fighting against her, particularly during her delusional operations. However, she failed miserably due to the fairly obvious appearance that her older sister is far stronger than she, assuming it to be punishing her swiftly for stubbornly accepting reality.
Rikka's father, however, it is learned that he had passed away when she was still too young to understand.
Her mother yet...
Shinka is her classmate and class representative, who is also a member of her own club. However, she formally joined to prematurely assuming it to be an act of getting close to Sanae and look for her "evidences" and attempt to destroy them for her sake. Despite being irritated by Rikka's chunibyo behaviour quite intensely sometimes, she'd willingly help her in her romantic relationship between herself and Yuuta.
They were seen to be formally meeting during an initial recruitment between Rikka and Yuuta themselves, the latter of which also wondered if they were interesting in joining her napping club (which she claimed that she had already formed before theirs, but failed due to lack of people's interest). Realising that there's no other choices, they were sent off by their teacher and given the task to clean out the unused dirty classroom in order to have it serve as a base of operations. From this point on, they form a particularly close relationship and show mutual respect to each other.
At first, Rikka initially viewed Satone as a threat to her relationship with Yuuta . As shown fairly early in the series, the duo would challenge each other to a delusional duel due to their mutual chuunibyou personas. However, they eventually became friends afterwards in the series, she'd also like to help her in improving her relationship with Yuuta.