Vovka used to be Sasha's best friend and was once a model student and talented artist. Case in point: last year, he painted a work called Comrade Stalin at the Helm (13.2), which won the school's art competition. Until quite recently, Sasha and Vovka hung out a lot, and Sasha "went to his apartment hundreds of times" (23.3).
And then "one day Vovka snapped" (13.2). His grades took a nosedive, and he started acting out (as it's called in polite society). Apparently, his new default mode of operation is to pelt kids with snowballs on the playground (hard!) for not toeing the Communist Party line, and to constantly disobey his teacher.
He's a troublemaker in the extreme, having all the stock and standard qualities of a garden variety bully. Vovka's described as having crazy eyes: he "looks at [Sasha] with eyes so scary" it causes him to step back (17.8). And he's also not above throwing around some pretty hurtful slurs to the other students. He calls Sasha "Amerikanetz," which is a slur meaning "American born," because that's where his mom was from, and that automatically makes her suspect. Nice guy, that Vovka.
So, why did Vovka go from Jekyll to Hyde all of a sudden? It turns out that there's a pretty legit reason for this. The big secret is that Vovka's dad was recently executed for being a "wrecker" (22.10). And when Nina Petrovna cruelly blabs this little tidbit to the entire class, Vovka goes nuts:
Before anyone has time to answer, Vovka flies at Nina Petrovna, grips her by the throat, and begins strangling her. Nina Petrovna's face turns red and her eyes bulge. She makes gurgling noises and starts kicking up her legs. (22.12)
Unfortunately, there's no sympathy for Vovka to be found in the society he lives in. Since his dad is an enemy of the people, it's off to the orphanage for poor Vovka, and there's nothing much anyone can do about it.
For starters, he's violent, where Sasha is not. Vovka doesn't shrink away from playing "firing squad" and hitting guys with snowballs (there's a pretty cool illustration of this free-for-all at 12.F1). Sasha, though, feels sorry that he broke Four-Eyes' glasses, and he only does so after he's provoked.
Also, Vovka is outspoken and irreverent, where Sasha's meek and mild. Remember how Vovka refuses to stop reciting the Pioneers pledge when the teacher tells him to (13.7-13)? He also "drops the sacred Pioneers banner right down on the wet floor" (17.8). This is way different from how Sasha shows his reverence for all things Pioneer- and Communist-related.
So are we just supposed to write Vovka off as a budding sociopath? We don't think so. There's some kindness buried somewhere underneath Vovka's little psychopathic heart, and he's not all bad after all. In the end, he sympathizes with Sasha, and they share a moment of bonding because they have one giant thing in common: they're both the sons of "criminals," as Sasha tells us, "Only when Vovka grabs me under my arm do I notice that I'm sliding to the floor" (23.15).
Vovka literally supports his former friend when the principal reveals that Sasha's father, too, is an enemy of the people (and will very likely die in prison). Then later, when Sasha thinks Vovka is turning him in for breaking off Stalin's nose, he actually accuses Nina Petrovna, saving Sasha's butt and dispatching the school villain in one fell swoop.