The Solar Guard is a military body that serves the Church directly. The Paladins are the elite of that military.
While most of the Guard acts as to secure the Church's properties and interests across the Galaxy, the Paladins--the Companions of the Pope--are those tasked to handle the most urgent matters, either directly or as a favor to another in the interest of Mankind or Christendom more broadly.
Each Paladin has a place at a chamber reserved for him. This is his Seat. Each Seat is named for a heroic figure of Man's past, such as Roland the Frank. Three of these Seats are named for the three traditional Archangels of the faith, and their holders are the seniors of this fraternity. This chamber is rarely used, as the duties of the Paladins often have then in the field, but when they do meet--and it is not to initiate a new brother--they do so at this chamber in the Guard's headquarters on Palatine Hill in Rome.
Each Seat is the seal for a Super Robot that shares the Seat's name. The holder may call it forth when authorized; the Archangels are trusted to do so without needing permission. They also have a highly-customized Real Robot assigned to them, often named for a weapon or other key item of the namesake. (This is why Durandal is Ramsey's Real Robot.) Between the quality of the work, and the skill of the pilot, these Real Robots have taken on enemy Super Robots and won.
The conventions for Paladins accord them noble status, styling them as "Lord (Seat)" formally; the Archangels are accorded as Peers of the Realm and are entitled to be styled as "Duke (Seat)" but most are used to being "Lord" (having held a lesser Seat previously) and let it go under most circumstances. e.g. Sir Ramsey Hennepin, Lord Roland of the Solar Guard. Paladins are treated as (naval) Captains within a military context; Archangels are as Admirals.
A Knight of the Guard that gets elevated into the Paladin Brotherhood leaves the formal Guard hierarchy permanently, becoming "unattached" and--on the rare occasions where they aren't on assignment--allowed to operate independently. Prior to that point, they enjoy a military career that often has them posted to a variety of posts; in this way, Paladin mentors groom potential successors and test their students' character over time.
There are no female Paladins, just as there are no female Guardsmen, and it is literally impossible to change this.