From Tokyopop:
Sergeant Keroro is the Captain of the Space Invasion Forces Special Advance Team of the 58th Planet of the Gamma Storm Cloud System, sent to the planet Pokopen (aka Earth) to collect intelligence for his planet's invasion force. He is also a frog. After his ship crash-lands in the planet earth, he takes shelter in the Hinata household, but the two kids, Fuyuki and Natsume, find him and take away his alien weapons. When his people discover that the Pokopenians are aware of him, Keroro is abandoned - left to fend for himself in this hostile world. But he's not alone - four other pre-invasion agents were are also lurking on Earth.

Note: Won the 50th Shogakukan Manga Award for children's manga in 2005.

This story acts as a sequel to the original "Dragon Quest Monsters" game for the Gameboy. Kurio, a child who wishes to be a hero, is thrown to a world where monsters are found all over, and is asked to become their new Monster Master hero since the great tree is withering, the monsters are going berserk, and Terry, the previous monster master, has gone missing.