Seikijin usually reside in egg-like cocoons until they're activated. To enter a Seikijin, a Seikishi places his or her palm on the cocoon. After the palm is read, the Seikishi is then encased in a sphere as he or she travels to the chest cavity. As soon as a pilot in the cockpit activates it, the Seikijin breaks from the cocoon, which then forms the armor (and tail if the Seikishi is powerful) depending on who is in the Seikijin. Normal people cannot be taken inside the cockpit due to the high amount of Ahou radiated inside.
A weakness for Seikijin is due to their human anatomy; they are vulnerable to pressure points, which was demonstrated when Kenshi paralyzed Cordyline's Seikijin using one of Wahanly's Kikkoujin. Another weakness is that if the pilot passes their operational limits or if one of the energy reactors (the rings on a Seikijin's back and forearms) either break or go out of control, the Seikijin will revert to its cocoon form. One other weakness is that if a Seikishi is too powerful, the Seikijin will break down and fall apart so you need to upgrade it so it doesn't. The last weakness is that if the Ahou in the air is too thin, the Seikijin will revert to its cocoon form, so you need a Ahou-cord to power it but the cords can be cut. According to Aura, Seikijin are a cost-effective alternative to raising armies or building heavy artillery to defend their homelands.
To repair a damaged Seikijin using Ahou, a Seikishi has to undergo being baptized by the Church to become a Seieishi in order to repair them. Otherwise, the damaged Seikijin has to be sent to the Church directly for repairs. To maintain the balance of power in each country, the Church can only supply a certain number of Seikijin to each country to prevent further power struggles.