For those who are new to this game, you'll wonder why in the world this is happening in this game. To fill in the gaps, this is the most prevalent theory why things are happening in this alternate period.
In 1918, the Kouma Wars, before the "Steam Evolution" (instead of Industrial Evolution), a huge demon appeared at Nihonbashi in Tokyo. It was sealed away by the Imperial Army's Counter-Demon Team, but their success came at a cost. Due to this incident, this effort was made to secure Tokyo from future spiritual threats, thus the formation of the Imperial Capital Defense Program and the Imperial Assault Force. This also begins the making of robot fighters using "spirits" to help make it move starting with Tokyo's kobus.
In order for the spirits to grow and fill the goodness to the hearts of people, the women has to learn to sing and act in a theatre and project their voices to the mass audience. To complete the group, a male commander gets involved in not only working the kobus, but also keeping the team together through the hardships he has to endure by going to the heart of the girl's problems.
The Flower Division is suppose to have started around 1924. Due to its success in taking down the huge demon, European Division with it's theatre called "Theatre Chatts Noires" made in 1926 with it's jinbus and Star Division with Little Lip Theatre made in 1928 with the STARs.
The most uniqueness about the kobu or in the game called the STARS is besides the need of spirits to run the machines, it also tailored for a person for its unique strength. For example, Sakura, Ichiro, Gemini and Shinjiro uses a samurai sword, therefore a large replica has been made to coincide with the persons strength. Cheiron uses a whip, Diana has psychic powers and uses a bird, Subaru uses double fan, Rosita uses double magnum pistols and Ratchet uses knives.
Also the most common aspect with the Flower Group, European Group and the Star Group is none are tied to an ethnic group. All girls comes from different parts of the world to come together, sharing personal issues and having the strength to overcome it in the kobu's to now STAR spirit machines. For example for this game, Gemini came from Texas, Diana came from Boston, Rosita came from Mexico, Cheiron came from Harlem, Subaru and you as Shinjiro came from Japan. So in other words, it really gives an international feel to it rather than making them all japanese, french or english.
If there's any difference between the STAR, jin-bu and ko-bu is simple. Jin-bu and ko-bu has to be transported and always has to fight in its current form. STARs special feature really is the ability to fly which is unique. Because they have to fight very giant robots, it makes sense for Mr Wong to use that extra feature.
Lastly, if you live in Japan, if you're curious about the aftermath, check the OVA series. Briefly it's about an Egyptian god taken emotional hearts from parts of New York to create his own paradise after Ratchet left to oversee the newly developed German Fighting Division. Comedically, Shinjiro has to play Cleopatra which pays off well at the end of a total of six episodes. Consider lucky if you get a chance to see it, if you come to Japan.