كيرو
"Sgt. Frog"s manga artist, Mine Yoshizaki, created the character concept designs for "Seven of Seven". As such, various crossovers exist between the series. The character of Mutsumi Hojo, who appears as "Examinee 623" and the host of his own radio program in "7 of 7", is a major character in "Sgt. Frog". Melody Honey, a minor character in "Sgt. Frog", appears and even receives a backstory. Additionally, the recurring ghost character in "Sgt. Frog" very closely resembles the design of the ghost that appeared in the temple in episode 12 of "Seven of Seven".