The speaker, as we can infer from the poem's first line, is the Windigo. The poem is told in the second person, the Windigo speaking in the "you" voice to the young child. Essential to note, however, is that Erdrich turns the table on traditional tellings of the Windigo story; here Erdrich tells the story from the Windigo's perspective, instead of from that of a removed narrator. In doing so, she grants us the unique vantage point of witnessing the poem's chilling action unfold through the Windigo's icy eyes, a character whose motivations we know to be, um, well, decidedly not chill.
At the same time, presenting the Windigo's perspective makes this poem more complicated than just a scary monster story. We get a real sense of longing from the beast in line 14: " Oh touch me, I murmured." Maybe this abominable ice beast is not so depraved after all? As with the ambiguous ending, we're never quite sure how exactly to feel about this monster. Pity him? Be totally skeeved out by him? Both at once?