Also Apostle of the Faith.
An elite soldier in the Azadi culture. Where the Azadi usually try to convert people to the worship of their Goddess via philosophical argument or cultural conquest, sometimes more desperate means are needed. The Azadi seem to believe, from comments by the Six, that a death at the hands of one whose faith in the Goddess is unquestioning, ensures the victim a rebirth in Her love and favour, whatever they did in life. A death in this manner is euphemistically called a 'conversion'. Helpfully, this belief also serves to remove powerful threats to the Azadi way of life.
An Apostle serves to fulfill this purpose, acting as part ambassador, part soldier and part assassin - given Azadi culture it is assumed that Apostles are usually male. From comments by Commander Vamon, it seems that Apostles are often of notable, if not noble, blood, and they are also highly skilled with the sword (presumably this includes martial arts as well), training in the War Garden in Sadir. Kian Alvane mentions he is not a 'scholar' and therefore does not think it is his place to question the actions of the Six; this implies Apostles are given a detailed but very specialised education. Once trained, Apostles are dispatched according to the direct orders of the Six.
If Kian's attitude is typical, Apostles are quietly confident in their actions because they believe they are both protected and guided by the Goddess. According to their beliefs, they do not see their duties as murder, rather as serving the individual they are targeting. However violent their actions, they are fundamentally in the victim's interests because they will then be reborn in the sight of the Goddess... one way or another.