山口勝平
is Ataru's classmate and best friend. He is essentially a background character and is often involved with Ataru's usual mischief. Though at times he seems to be in love with Lum (like all of the male students), he has a girlfriend of his own and mainly just serves as a voice for the rest of the class. Kosuke does not appear in any of the anime adaptations, where his role and lines are given to Lum's Stormtroopers, usually Perm. His name comes from the then chief editor at Shogakukan . He is voiced by Kappei Yamaguchi in the new OVA.