The main heroine, she made a contract with Togashi Yuuta. She is an active Chuunibyou, who still has the syndrome. Her delusional syndrome involves her right eye which she calls it her "Wicked Eye," in which according to her it wields darkness tamed by the power of a pure heart. Actually, her so called "Wicked Eye" is nothing more than a colored contact lens which she conceals underneath an eye-patch. Despite being quite attached to Yuuta, she is cautious of strangers and adapts a battle pose whenever she meets someone for the first time. She is also rather clumsy, often tripping over and forgetting things. She also doesn't have many friends, as shown that in her mobile phone there are only her sister's email. (Source: Wikipedia)
Rikka's father died in the past and Rikka stubbornly denies that her father had died. At the process, she accidentally met Yuuta, who was an active chuunibyou at that time, and was amazed by how cool Yuuta was as the Dark Flame Master. And that's the cause of her chuunibyou, she believes her father is in the "Horizon."
As the story develops, Rikka eventually falls in love with Yuuta.