Stories told in the first person are easy to identify. The narrator will call him/herself "I" or "me." In "The Scarlet Ibis" the first-person narrator is known only as Brother. Brother is what his brother, Doodle, called him. The story is his memory of their time together. Brother is a central narrator because he's a main character in the story. Brother tells us what happened to himself and his brother. His actions play a major role in shaping the events of the story. Notice that we say Brother is a main character, not the main character. We think he and Doodle share the starring roles, but some readers feel strongly that Doodle steals the show.
That settled, let's to figure out where Brother is coming from. Now, we know that Doodle lived from 1911 to 1918. What we don't know is how much time has passed between then and when Brother is telling the story. Brother says, "It's strange that this is still so clear to me, now that that summer has long since fled and time has had its way" (1.2).
This sentence suggests that Brother is much older than he was when Doodle was alive. But, it gives us no idea of how much older. It could be years or it could be decades. Some readers think he's still a young man living at home; other thinks he's an older man who inherited the house from his parents; still others think he's somewhere in between and just home for a visit.
Not knowing exactly where Brother is in time gives the story more of the feel of memory. Does it matter how much time has passed? If we knew, would it help us decide whether Brother is being honest with us, or whether his memory is as "clear" as he says it is? Regardless, Brother will probably remember his time with Doodle all his life. We might also agree that Brother blames himself for Doodle's death, and that he's extremely hard on himself. We also think that Brother really loves Doodle, and did back then, too. He's coming at us from the grief, guilt, and love.
As such, we can say that Brother might seem to be a reliable or trustworthy narrator because he seems to be expressing these emotions sincerely. But, he's also unreliable because his memory is colored by his grief, guilt, and love. We've all heard warnings not to say bad things about dead people. Brother seems careful not to say anything bad about Doodle, and careful to say as many bad things about himself as possible.
It's easy to get caught up on Brother's guilty feelings and forget that he was a confused kid in a difficult situation. It's up to us to remember this and try to see beyond his point of view. We can remember that he was only six when Doodle was born, and only thirteen when Doodle die. Should we judge him (as he does himself) as we would an adult in the same situation?