Keroro is the titular character and protagonist of the manga and anime series Sgt. Frog, (also known as Keroro Gunso).
Keroro is a Keronian, member of the dominant species of Keron, the 58th planet in the Gamma Nebula. Keroro's father was known as a legendary sergeant, and Keroro, himself, still in his early days in the army, became known as a legend himself. However, most of his success seems to have come from various lucky coincidences, rather than personal ability. One of the reason Keroro becomes famous is the way that he gains great boosts of ability in the presence of enough humidity, but when he gains that power, he becomes too unstable to use it well.
Keroro was chosen to lead a platoon that would be part of the invasion force launching to the distant planet Earth, which was known as Pekopon (Pokopen in the manga) by aliens in general. By pure chance, Keroro was involved with stopping a surprise attack launched against Keron's army at the time.
Shortly after arriving on Earth, the platoon members spread themselves in a recognizance mission in Inner Tokyo, a fictional city in West Tokyo, which soon ended up in disaster. Keroro, Tamama and Zeroro had various problems and were found out by humans, which resulted in the main force of the army abandoning the mission, leaving Keroro's platoon stranded on Earth while the army's main force returned to Keron.
Meeting the Hinata Family
Keroro ended up found by two human children, occult fanatic Fuyuki Hinata, and athletic genius Natsumi Hinata. Although initially hostile to them, they soon adapted to each other, and Keroro stayed hiding with them, in exchange for helping with household chores. Meanwhile, he attempted to regroup with his platoon and start new operations to conquer Earth.
In the years following that, although having little success with an actual invasion, Keroro gathered his platoon member, recovered support from the Keron army, built up his arsenal with stronger robots and vehicles, stopped competing alien invaders and discovered ancient weapons left on Earth by previous invasion attempts of the Keronian army, often having to destroy them due to out of control autonomous mechanisms.
Keroro used to be known as a good officer, but since his arrival to Earth, that image has been completely destroyed. He prefers to spend his time making Gunplas, watching anime (especially the Space Captain Geroro series, a parody of Space Battleship Yamato), or reading manga, when he is not doing the house chores of the Hinata household.
Keroro is carefree, negligent, greedy, vengeful and a coward, with a short attention span for anything not related to his entertainment, although he occasionally displays bouts of unselfishness and bravery. He often misspends the invasion budget on models and he always leaves all the paperwork for their bosses back in Keron up to the last possible moment, often ending up punished for it. He is also incompetent and bumbling than his recruits
His invasion plans often hide personal goals, like getting back at Natsumi, and even when they do not, they are often harebrained ideas without a chance of success. His plans for raising money usually follow the same pattern. The few plans that do have a chance of success usually end up getting out of control and possibly causing more destruction than intended, needing to be stopped, or are considered illegal by the Space Police.
Unlike the other members of his platoon, Keroro doesn't have any particularly advanced skills in any area. When he faces humans, Keroro often uses "Nyoros", small one eyed spaghetti-like creatures that wrap his foes. He has been briefly shown wielding firearms, beam sabers and shields, but not with any particular talent. He's also able to fire beam attacks from his eyes, although it took a long time for him to train the later adequately, and, even after his training, it remained a flawed technique that could be easily dodged by standing still.
Most of his longest participations in battles, especially in the anime, come while piloting robots or spaceships, rather than actually fighting directly.
As the leader of the invasion force, Keroro had the KeroBall, an advanced Keronian weapon that could make one's will into reality - able to realize basically any effect, from creating an underground base to brainwashing to creating a large pocket dimension used as a training facility. However, Keroro couldn't control it well, often resulting in unintended effects while attempting to use it. Keroro also soon lost it to Fuyuki Hinata, who keeps the Keroball most of the time afterwards.
Occasionally, when there is a lot of moisture in the air, Keroro can go to the state that helped him become famous "back then", increasing his strength and speed, and even giving him power of flight. He's also able to release a powerful beam attack when under this form. However, that usually ends with a Nyororo, an alien creature that absorbs moisture, sucking off all the water in Keroro's body. In the few times where that didn't happen, he's just too unstable to actually do anything useful in that state.
Eventually, Keroro learned about artificial dragons made by the Keronian army in the ancient past. Keroro and his platoon gained the ability to transform into a giant Dragon with the help of Shion, a human girl able to read an ancient Keronian text. In that form, Keroro is able to release energy breaths and fly at high speeds, both under Earth's atmosphere and space.
In the manga, it's revealed that Keroro's proof of leadership, the Keron Star, the yellow star on his belly, has a similar power by itself, theoretically allowing Keroro to take Dragon form at will. However, due to his weak will, he's unable to do that by himself most of the time, needing help from Kaneami, a human girl who was altered by aliens and can reveal someone's potential.
Aside from his personal abilities, Keroro has access to many highly advanced space tools, both exclusive to his platoon, made by the genius Kururu, or just common space tools that are almost magic compared to Earth's technology.