Zoruru

Zoruru is a gray Keronian frog whose left side has been replaced with metal implants, his left arm has an extendable blade, which is his primary weapon. He has an artificial eye in the place of his right eye, and the entire left side of his face is covered by a metal plate. For current unknown reason, he holds a grudge against Dororo and seems completely focused on getting revenge. During his platoon's invasion on Pokopen/earth he destroyed Mutsumi and Koyuki, as an attempt to provoke Dororo to come forth; however, Dororo revived them later by using his ninja powers to stab their ship. Zoruru finally meet up with Dororo at the end of the fight between the Keroro and Garuru platoon and Zoruru reveals that he never held much interest in the Pokopen(earth) invasion to begin with, only wishing to seek out Dororo in order to fulfill his revenge. Sadly, Dororo holds no recollection as to whom Zoruru is and tries desperately to remember his name, mistaking him for Girara, Dasasa, and Tosasa; assumed to be his former classmates. This causes great disappointment to Zoruru, as he wasn't even recognized by the very character who is well known to be ignored by many and he slowly fades away. When the Garuru Platoon leaves, Zoruru is in a position which is seen many times, Dororo's trauma switch.

After this episode, there is a running gag for him to be forgotten in one form or the other; such as, when he is the only one left out without an introduction in the special chapter of volume 13.

In 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 appareance in volume 13. Source - Wikipedia