Sgt. Frog Episode 186: Arisa vs Ghost, Aggression is the Lack of Progress
Episode 186 of 358 of Sgt. Frog, aired November 10, 2007, 25 minutes. Available with English subtitles and dub where released.
Episode summary
Thinking it will make a good excuse, Keroro makes up a yokai called "Invasion Goes Nowhere," and blames it for their failure to invade the planet Earth. But when he starts impersonating said yokai to play pranks on the others in his platoon, he gets an unexpected visitor -- Alisa, who wants to hunt him as one of those that lurk in the darkness! Source: crunchyroll