Sgt. Frog Episode 57: Viper Rash!
Episode 57 of 358 of Sgt. Frog, aired May 6, 2005, 25 minutes. Available with English subtitles and dub where released.
Episode summary
Viper`s older brother has come to Earth to get revenge on the Keroro platoon for killing his brother. Unlike his little brother, he doesn`t have a weakness for tokusatsu shows or a tendency to follow all of their cliches! Or does he? Source: crunchyroll