The series will end with Yasuna and Sonya having a battle, and Yasuna shall win
Yasuna is the adopted child of Kirika and Mireille.They did not train her how to kill people; but they told her at a young age what they used to do for a living. This is why encountering a child assassin doesn't surprise Yasuna at all. (That's what Mommies used to do!) Sonya's organization is not officially a rival to Soldats; but strong enough of an independant faction to make their school enough of a Truce Zone so that it was decided to be the safest place for Yasuna to go to school. One of her parents tend to watch from a distance to make sure nothing really serious happens.
Sonya is RussianShe is clearly a victim of Phenotype Stereotype, her name is, while not common, used in Russia and, finally, there is opening music.
Yasuna is Yui's (K-ON!) lost sister
Agiri spends her off-screen timeoperating a beach restaurant.
Yasuna is an assassin using Obfuscating Stupidity.This explains why Yasuna attempts to be close to Sonya all the time no matter how much she proves to be extremely dangerous. Sonya suspects this (but has no concrete evidence), causing her to find any small opportunity for an excuse to beat Yasuna up. She is just waiting for the moment when Yasuna's assassin self-defense training kicks in and proves her status as an undercover assassin.
Yasuna is immortal.In one of the episodes of the Anime, when walking to school Yasuna slips on some ice and accidentally yanks Sonya's scarf, to which Sonya punishes Yasuna by yanking her scarf up the branch of a tree, thus hanging her. And yet she walks around school as if nothing has happened. Now, to thorougher support this theory, let go the the Mange, specifically, chapter 59: In which Yasuna tries to move Sonya by showing her a Sakura blossoming and says "If you're not moved, I don't mind being buried under the tree." Well enough,Sonya holds her to her word. And yet in the next chapter Yasuna is back unharmed, despite the fact she couldn't dig herself out. And lastly, if Yasuna was mortal, would she really make such a bold statement like that? More importantly, would she keep coming back to Sonya if there was such a great risk. There are many times in the anime and mange were Yasuna is rendered unconscious, but what if she actually gets put into her equivalent of death only to come back to life moments later. This form of immortality also gives Yasuna a healing factor, which would allow her to recover from her injuries quickly.