Alyona is the pawnbroker Raskolnikov murders...and probably a major reason that you don't hear the name "Alyona" thrown around a lot. "Oh, nice to meet you, Alyona. Were you named after anyone special?" "Yeah, my parents were Dostoevsky fans..."
And, like Pyotr Petrovitch Luzhin, Alonya is not too fleshed out. But, because she's a character written by Dostoevsky, she sure isn't 2-D, either.
We can sympathize with her because she's a woman in her late 60s who's pretty much alone, because she's helpless before Raskolnikov and his nasty axe, and because she's killed in a brutal and cruel way.
But, on the other hand, we know she cheats poor people and beats Lizaveta. Still, we probably wouldn't sentence her to death for those acts. She's a brilliant choice of victim, from a fictional standpoint—a cruel person but probably not one of the cruelest people in St. Petersburg. (You could have chosen a better mark, Raskolnikov.)
Because she's so helpless, so unequal to Raskolnikov in strength, it makes his crime harder for us to understand than, say, if he'd killed somebody that had a chance against him. By making his crime against her so brutal and cruel, Dostoevsky is adding yet another layer onto the layer cake of Raskolnikov's character. (But it's probably not the kind of cake you want to serve at your next b-day party.)