As little as we know about the figure in the portrait, we know even less about this poem's speaker. Given that this poem is a bit of a blazon (see "Form and Meter"), we might go out on a limb and guess that he's a man, but even there, we don't have much proof.
But as with the racial and gender ambiguity in "Portrait in Georgia," the ambiguity of the speaker serves a purpose, too. While it may seem like this speaker is referring to one specific figure, he's talking in universal terms. This lynching scene happened thousands of times in the old South, and each time there were stories that were never told, dynamics that were never understood, justice that was never truly done. The speaker's lack of a specific identity only adds to the universality of this story.