My name is Sasha, I was found in the Metro tunnels by a man named Artyom, my father. I am unlike everyone else of the Metro, for I am not human... but a pony.
Alyona is a Stalker - she risks her life frequently in search of loot in Moscow, or the Dead City, as some call it. After finding a mortally-wounded friend in Dry Station, they stick together until an anomaly whisks them from the Metro to Equestria.
Light and Darkness are easy to distinguish. Many think issues are either Good or Bad. I used to think that way as well. Since then, I've learnt that nothing is simply Black or White.