A trusted bro, that would, if needed, go to battle for you. This is especially applicable when said bro could be considered an outsider or iconoclast.
The term is inspired by Bucephalus, the trusted steed of Alexander the Great. He was originally considered an untameable horse but helped Alexander carve one of the largest empires of all time, spanning from Greece to India.
"Whoa dude, don't hate on my brocephalus! He had my back last week when we scuffled with some B&T at Webster Hall."
"I dumped Jenny, so I am going to go watch 2012 with my brocephalus tonight."