Zoruru

ゾルル

Zoruru is a gray Keronian frog whose left side has been replaced with metal implants; his left arm features an extendable blade, which serves as his primary weapon. He possesses an artificial eye in place of his right eye, and the entire left side of his face is covered by a metal plate. For currently unknown reasons, he holds a grudge against Dororo and appears completely focused on seeking revenge. During his platoon's invasion of Pokopen/Earth, he destroyed Mutsumi and Koyuki in an attempt to provoke Dororo into appearing; however, Dororo later revived them by using his ninja powers to stab their ship. Zoruru finally meets Dororo at the end of the fight between the Keroro and Garuru platoons, revealing that he never had much interest in the Pokopen (Earth) invasion to begin with, wishing only to seek out Dororo to fulfill his revenge. Sadly, Dororo has no recollection of who Zoruru is and tries desperately to remember his name, mistaking him for Girara, Dasasa, and Tosasa, who are assumed to be his former classmates. This causes great disappointment to Zoruru, as he was not even recognized by the very character well known for being ignored by many, and he slowly fades away. When the Garuru Platoon leaves, Zoruru is in a position seen many times: Dororo's trauma switch.
After this episode, there is a running gag of him being forgotten in one form or another; for instance, he is the only one left out without an introduction in the special chapter of volume 13.
In the Tokyopop version of volume 10 of Keroro Gunso, the letter R was written backwards whenever he spoke. This was, however, dropped with the following volume and his brief appearance in volume 13. Source - Wikipedia