Kildren are genetically modified humans that stop growing in their late teenage years and up to their early twenties. They are not able to die from anything and are immune to all diseases, hence they can only die in battle. Brutal air battles tend to keep them from lasting long, since they do not gain very much experience. Rostock seems to consider most of them as expendable fighter pilots and soldiers. However, they can be reincarnated as a clone of their previous selves and retain their abilities, similar personality, behaviors, and even appearance, with perhaps only slight changes.
Kildren were created by a Rostock genetics project directed by Tohochika Mozume, who oversees the first known deployment of Kildren at Hariyu base in Sky Crawlers: Innocent Aces. According to the novel The Sky Crawlers, Mitsuya tells Yuichi Kannami that Rostock was developing a genetic control agent product when they somehow managed to create the Kildren. Kildren was going to be the product name for the control agent, but Rostock instead decided to use it on their new creation of children that do not die, such as Yuichi Kannami and Mitsuya herself.