The Boros guild combines white's community devotion (and occasional autocratic over-zealotry) with red's emotional fervor (and occasional blind rage) -- a potent and potentially scary combination, especially given the fact that the Legion holds the position of law enforcement in Ravnica. It is a guild about combat, more than all others -- but combat to defend and preserve the higher law spoken down from its archangel guild-leader, Razia, Boros Archangel.
The look and feel of Boros stems from its militaristic nature -- its creatures are heavily armored from head to toe, yet decorated in sun-symbols, reflecting the guild's power structure and devotion to the angel at the top. Here are concept sketches from the Ravnica style guide for a Boros paladin and foot soldier.
Boros paladin and foot soldier
Boros doesn't just employ humans and fighter-types, however. Its military order applies equally to its wizards and clerics, and more exotic beasts such as its order-following (but still combat-feral) giants, minotaurs and rocs.
(Clockwise from upper left) Boros cleric, wizard, roc, giant and minotaur
The guild-leader, Razia, Boros Angel, is also depicted in the style guide in this Tomas Giorello sketch.
Razia, Boros Angel concept sketch