What are the feathers on Rei's neck? As of episode 7, we know the following:


Rei is somehow connected to the Alones that are attacking the city.
The (unnamed) black bird is able to inflict extreme pain on Rei through those feathers.
Every time Rei fires an arrow at an Alone, she consumes one feather.




Is there anything else that has been revealed? Do these feathers symbolize anything?
As of Episode 7, it's been revealed that it indicates how many Empowering Arrows she can fire and that she only has twelve of these arrows available.