Okay I'm gonna talk about the Ruhe now
Ruhe are a counter part to the Grimm in RWBY, they in fact fight them alongside the one who can call upon them (in this case Dryr)
their name means Calm in German (this is true as i was informed by :iconlexalice: when i asked her of it) and is the opposite of Grimm which can mean Fury in German. Also their are only 13 of them in summon-able batches (their are two Ruhe Summons that have more than one Ruhe) and they are each one of a kind and there are no others like them. That being said they can only die if they themselves stay after they would otherwise be forced to leave due to injury, otherwise they return to their world and have to recuperate after a fight and cannot be summoned again for at least a day.
one very note worthy detail about them is that they cannot be used against anything besides Grimm and other wild creatures (this is only if they pose a threat to their Summoner) in the RWBY Universe, the exceptions are three of the 17 individu