The opposite of "narrow" that starts with the letters "Br" is "broad." "Broad" refers to something that is wide or large in extent, contrasting with the limited width implied by "narrow."
brunt
It could be brio, bred, brag, brew, brie...
brick brim broth brat brake brawl.............. oh and br br br br br
brittle
Brahmaputra
Brisk.
There is no antonym of the noun "coward" that starts with BR. The adjective BRAVE is the opposite of the adjective "cowardly." The noun "brave" refers to Native American male warriors. The secondary definition "people who are ready to face and endure danger or pain" is spurious and is not used for that purpose. The generally accepted antonym for coward is "hero" (a brave or courageous person).
The opposite of slow starting with "br" is "brisk." Brisk is defined as quick and energetic in action or movement. It is a term often used to describe a fast pace or speed.
Brook
Brain, breast and bronchi are body parts. They begin with the letters BR.
brawd
· Barium (Ba) · Berkelium (Bk) · Beryllium (Be) · Bismuth (Bi) · Bohrium (Bh) · Boron (B) · Bromine (Br)