The question of Doodle (from Hurst's short story "The Scarlet Ibis") being the antagonist to the narrator can be answered in two different ways.
First, Doodle could certainly be identified as the narrator's (Doodle's brother) antagonist. Typically, the antagonist of a text is the character with whom the protagonist has some type of conflict. Based solely upon the fact that Doodle and his brother are the two main characters, and the narrator is the protagonist, Doodle would be considered the antagonist.
On the other hand, one could look at the narrator's "issues" with Doodle (internal conflict--which is a conflict within one's self) as existing as the "antagonist." Therefore, if looked at in this fashion, Doodle would not be the antagonist, the narrator would.