Woman at Lubyanka Prison
Although she is not named, this woman that Sasha meets while standing in line to see his dad at the prison ends up being one of the kindest characters in the novel. We find out that she's waiting to see her son, who was arrested (and really: at this point, who hasn't been arrested?).
She takes pity on Sasha for being cold and hungry, and does her best to remedy that by letting him wear the warm scarf she's made for her son, and a warm potato. She also offers to let Sasha come sleep in her son's (now empty) cot.
In a nutshell, she's willing to stick her neck out for a stranger, when Sasha's own aunt and uncle won't do the same because they're so afraid of the consequences. That tells us that there's at least one kind-hearted person left in this cruel world.