WARNING!! Keep an eye out for
spoilers! These are thorough reviews so...tread carefully.
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The Main Characters

Seita
Seita is a 14-year-old boy who is suddenly expected to play the guardian of his younger sister, Setsuko. Throughout the movie, he is thrown into situations that normal 14-year-olds would never face - situations where he has to find a new place to stay, and where he has to comfort a hungry sibling when there is no food to eat.
In many ways, Setsuko's death could be blamed on Seita. His only saving grace is the fact that he is still a child - it is cruel to place all the blame on someone who couldn't suddenly be expected to become a perfect, responsible, and capable adult.

Setsuko
If you love cute little kids, you'll love Setsuko. She's Seita's 4-year-old sister. Her childish innocence throughout the movie was probably one of the few things that allowed Seita, and probably the audience, to keep going - without her the whole story would have been just one long, dark tunnel.
What I found most surprising was that fact that Setsuko didn't complain very much - she was probably hungry every day after she lost her home, lonely and always playing by herself while her brother went out in search of food....
Her story is very sad.