We don't know too much about the speaker of "Helen." We don't know if the speaker's male or female, but we'll refer to the speaker as "she," just to keep things simple.
What we can say about the speaker for sure, though, is that she's very detached. Her language is spare and direct. We feel sometimes like we're getting a list of facts ("All Greece hates," then, "All Greece reviles"). And the basic horribleness of these facts—that Greece can't stand Helen through no fault of her own—is enough to make us empathize with Helen. We feel for her, not because the speaker asks us to, but because we just can't argue with the awful truth.
Our speaker is calm, cool, and collected, and yet she still inspires strong feelings in her readers. She's pretty powerful, if you ask Shmoop.