Sgt. Frog Episode 121: Dororo - The Lady Ninja Karara Makes Her Appearance!
Episode 121 of 358 of Sgt. Frog, aired August 4, 2006, 25 minutes. Available with English subtitles and dub where released.
Episode summary
Karara returns, having trained to become Dororo`s bride! But Mecha Viper is back, too. When Karara mistakes Viper for Dororo, she starts begging him to teach her ninjutsu! Source: crunchyroll