Tetrominon said: There's probably a more graceful way to translate 盟友 here.
There really isn't, when it's being used as a form of address. People have been trying since MoF came out and all the alternatives I've seen are inferior.
Nitori: "Hyui!? Comrade! The launching ceremony for the new submarine is about to begin!! We can still see it if we hurry, so get on board the Ketten!!"
"ひゅい?! (Hyui!?)" is Nitori's distinctive yell originated in Subterranean Animism.