The trio being called the "Knuckleheads" (also "Bonklers", "Boneheads" or "Numbnuts" depending on translation) started when Kagura called Tomo and Osaka "Bonkurāzu", however, Osaka did not initially believe this included her because she thought Kagura had only been speaking to Tomo and saying "Bonkura" which is singular. The three of them tend to stick together so as to not be alone in being underachieving in a prestigious high school. Particularly Yomi finds the three of them ridiculous.