Noun
brother-in-arms (plural brothers-in-arms or (rare) brethren-in-arms)
comrade in arms (fellow combatant or soldier)
Coordinate term: sister-in-arms
Lindelof, Pizzolatto’s unofficial HBO brother-in-arms, seemed to co-sign the creator’s pitch in his own Instagram post. Source: Internet
He also has some encouraging words for his brother-in-arms: “You don’t have to be Michelangelo, you just need to express yourself. Source: Internet