Soldier Frog (ケロロ ぐんそー Keroro Gunsou, lit. Sergeant Keroro) was a working concept for Keroro Gunso by Mine Yoshizaki, developed in 1998 before the main plot was finished. Many of the ideas of the original concept had already changed by the time the pilot chapter was published, and even more elements would change before the beginning of the series proper.
The initial concept for the series focused on the idea of "mascot characters" and "cute girls" as a set of duos.
- Sergeant Keroro: Alien sergeant and mascot character of the series. It should be noted that a "Keroro Platoon" and the invasion aren't mentioned in the early material, so the relationship between the different alien frogs isn't known. He carries the same de arimasu (であります) speech tick used by his final version.
- Jun Yamato: An early version of Natsumi and 12-year-old girl who attends an all-girls' school. Her partner is Sergeant Keroro. She is an ordinary girl, but is quite distrustful. She likes sports and belongs to the handball division of Hinomaru Girls' Secondary School.
- Corporal Kekero: The early version of Giroro. He has a star symbol on his hat, rather than a skull and speaks with a Tosa dialect. He has a scar crossing through his chest, not just face.
- Chinatsu Yahagi: She is Jun's rival and Corporal Kekero's partner.
- Master Sergeant Rororo: The early version of Kururu. Unlike Kururu's casual speech pattern, he ends sentences with a polite Douzo (どうぞ) (By all means, you are welcome, feel free). Early sketches already show his spiral emblem, but the first finished design carried no chest emblem at all.
- Haruna Hibifu: She wears glasses and has a messy hairstyle. Haruna is an otaku, and seems to have multi-hobbies, such as watching anime, playing games and listening to the radio. Her partner is Master Sergeant Rororo.
- Private Second Class Roroke: The early version of Tamama. He shares Tamama's Boku (ぼく), a childish "I". He also extended the sū (すぅ) ending of his sentences, like Tamama, but didn't seem to focus specifically on desū (ですぅ). None of the sketches hint at Tamama's hidden darkness either. His hat was pink colored.
- Fuyumi Yukikase: An adult woman with a glamorous and sexy appearance. She wears a biker suit and is a street racer, boss and a sister. Her partner is Private Second Class Roroke.
- Lieutenant Rokeke: The early version of Dororo. He sported a black suit covering most of his body, alongside a gas mask and a dark blue version of Keroro's hat, although already with a four pointed star emblem. Some of those design elements would make to the series as part of Dororo's past as Zeroro.
- Akie Asagiri: Details about this girl are unknown, although she dressed in a traditional Japanese style. Her partner is Lieutenant Rokeke.
The pilot chapter published by Newtype already had several differences from the original concept for the series. Natsumi (Jun Yamato) and her best friend Mariko (Haruna Hibifu) record the metamorphosis of an amphibian for a summer assignment. However, that was no ordinary tadpole, as it shortly evolved to become an extraterrestial frog, a Batrakos, named Sergeant Keroro, shocking both Natsumi and Mariko. Keroro offers to repay them for saving his life, and they continue their assignment during that unforgettable vacation.
Differences between the original concept, the pilot chapter and Keroro GunsoEdit
- Corporal Kekero was changed into Corporal Giroro.
- Master Sergeant Rororo was changed into Sergeant Major Kururu.
- Private Second Class Roroke became Private Second Class Tamama.
- Lieutenant Rokeke became Lance Corporal Dororo. He also dropped his gas mask and Keronian hat before his proper debut in the series, while Rokeke was meant to sport it as his main design.
- Jun Yamato was later changed to Natsumi in the pilot chapter and finally Natsumi Hinata in the series proper. Her age became 14 rather than 12 and her main partner Giroro rather than Keroro.
- Chinatsu Yahagi, Giroro's original ´partner has several similarities with the anime's Rie Imogo.
- Haruna Hibifu likely became Mariko, Natsumi's friend who still appeared in the pilot chapter and made it to some later flashback chapters.
- Fuyumi Yukikaze was the basis for Aki Hinata, although going from an older sister character type to a mother.
- Akie Asagiri's design ended up as the basis for Omiyo.
- The original Batrakos are known as Keronians in the series proper. However, the Keroro from the pilot chapter and the Batrakos name itself was later reintroduced in the manga as bio-robots based on Keroro's data.
- All the human partners were female in Soldier Frog, but in Keroro Gunso there are both female and male partners.
- Hinomaru Girls' Secondary School became Kissho Academy.