The Russian word 'Comrade' devolved from the barbarian greeting 'Cumreach.' 'Cumreach' itself being a contraction of the phrase, 'Come within my sword's reach. I have no desire to kill you today.'
'Cupmate' is one of the most commonly used terms. Its nuance reflects similarity to the colloquial English term 'drinking buddy.' Implied to be pleasant company in public, but neither obligated to fight at one's side nor trusted to do much beyond avoid major public embarassment.
'Shieldmate' is one of the more focused terms. It implies one who is trusted to fight at one's side at that particular moment against that specific opponent. There are no connotations of long-term trust or any particular affection. An 'enemy of my enemy' friendship at best.
'Knifemate' is a rarely used term for a rarely found relationship. Reserved for the deepest of intimates, it physically describes one trusted with a knife at one's back in the dark. Multiple tragic sagas have been written about knifemates who did not live up to the term."