Here's a link to a good brief summary on Wikipedia
Brother is the lead protagonist of the story and also the narrator. He is not given a name but is referred to by Doodle, his brother, only as "Brother." He is six years old when Doodle is born. Brother has a high opinion of his own ability to run, jump, and climb, and wants a brother with whom he can share these activities. When it becomes clear that Doodle is capable of little more than lying on a rubber sheet and crawling backwards, Brother grows ashamed of Doodle's limitations and regularly taunts him. Though Brother loves Doodle, the love is tainted with cruelty and embarrassment.
Doodle is the mentally and physically retarded younger brother of the narrator, Brother. His family initially calls him by his given name, William Armstrong, but Brother nicknames him Doodle. Doodle's real strengths lie not on the level of his physical prowess, but on a more subtle inward level, to which Brother seems blind at the time the action takes place. From the beginning of his life, Doodle defies death and refuses to recognize the coffin that Daddy builds for him as his own. He shows a sense of wonder and respect for the natural world, crying with wonder at the wild beauty of Old Woman Swamp.
The other characters are Daddy, Mama and Aunt Nicey.