Seikishi (聖機師?, lit. "Holy Machine Wielders") - also known as Sacred Mechamasters - are the guardians of the Holy Land and the only people qualified to pilot Seikijin. Most of the Seikishi are predominantly female, though there are some male Seikishi (such as Dagmeyr, Cliff, Neil, Alan, and Ceres) and they're considered rare in numbers. Female Seikishi have the freedom of marriage and relationships if they fulfill their duties, which is to produce more Seikishi breeds. Male Seikishi, however, cannot due to their scarcity and are usually forced to engage in marriages of convenience, and in ancient times, the semen of a male Seikishi was a high-priced item.
A Seikishi who can repair Seikijin using Ahou (like Mexiah, for example) is called a Seieishi (聖衛士?, lit. "Holy Protectors"). To obtain this ability, a Seikishi has to be baptized by the Church. Unemployed Seikishi, like Cordyline, are called ronin Seikishi. Some female Seikishi wear unique tight-fitting (and sometimes revealing) bodysuits underneath their clothing that protects them from the Ahou emitted in the Seikijin's cockpit and the Doukatchu.
The main factor that distinguishes a Seikishi from a regular human is their ability to endure proximity to Ahou energy, which is used to power the Seikijin. Ulyte explains that, while fighting skill can be enhanced through training, a Seikishi's level of endurance to Ahou energy is something they are born with, and therefore cannot be changed. Seiskishi are limited by the amount of time they can endure an Ahou field, called the operational limit. Seikishi cannot pilot a Seikijin for longer than their operational limit without experiencing severe flu-like symptoms.