With the brother of Doodle as the narrator, his tone seems nostalgic:
It’s strange that all this is still so clear to me, now that that summer has long since fled and time has had its way....
and it is also rather rueful:
and now if an oriole sings in the elm, its song seems to die up in the leaves, a silvery dust.
The brother must feel responsible for what has happened to Doodle, for he was peevish with his brother, expecting too much from him. The final lines clearly convey his feeling of regret as poor Doodle lies bleeding on the ground:
I began to weep, and the tear-blurred vision in red before me looked very familiar. “Doodle!” I screamed above the pounding storm, and threw my body to the earth above his. For a long, long time, it seemed forever, I lay there crying, sheltering my fallen scarlet ibis from the heresy of rain.
Here the brother relives this tragic moment, regretting his peevishness with Doodle, poor little Doodle, who tries his hardest, but is disabled. The brother also rues that he could not do more for his brother, that his will could not conquer fate and somehow change the way things have happened as it has before, and make Doodle keep up with him. In a sense, the brother's expectations that no matter what, Doodle could do things, causes his brother's death. The brother, therefore, feels a sense of guilt and regret.
In the Scarlet Ibis, there seems to be a melancholic, haunted mood in which the whole story revolves around death, bad luck, graveyard flowers, etc. In the end, Doodle dies in a tragic way. There is also great tension between Doodle and his brother, in addition to his brother's inner tension.