A religious sect traveling the galaxy, trading goods and spreading the good word of their philosophy and scriptures. Never aggressive, they are known as trustworthy vagabonds and fair traders. However, in a galaxy of malevolent empires, criminal syndicates and pirate bands, they picked up the meaning of the word "defense" to a whole new level.
Their ships are equipped with best shields and armor plates, form capital temple-ships to quick escort interceptors. Driven by age-old sense of right and wrong, they decide collectively not to tolerate Empire's tyranny.
As such, they are a great and much needed addition to Rebel Alliance's liberation forces.
Design-wise, I love the imagery and shapes of Wild West settler carriage trains with their distinctive canvas barrel-shaped tarpaulins. Also the shape lends itself quite nicely to a defensive posture of craft. Armadillo or lobster-tail form is present in all cultures as a default organic armor shape.