What are their nicknames?
Koichi’s most common nickname is Kochan. Apparently that’s from Nagase or Tsuyoshi before, then fans started calling him that until now. Tsuyoshi’s nicknames are Tsuyo, Tsuyopon, etc. Fans usually call him Tsuyoshi-kun. Tsuyoshi calls Koichi as ‘Koichi’ though. The same goes for Koichi, calling Tsuyoshi as ‘Tsuyoshi’ or ‘Tsuyoshhhhi’ *LOL* without any -san, -kun, or whatever after the name. In Japanese culture, only the family or really close friends may usually call one without any suffix after the name. During their childhood, Koichi’s nickname was Domocchin and Tsuyoshi’s nickname was Domorin.