The speaker is never named, but he has a lot in common with Shelley. It's never a good idea to confuse a poet with a speaker, but this guy's mourning the loss of a beautiful youth, just as Shelley is mourning the loss of Keats. Like Shelley, the speaker also takes a harsh stand against unfair criticism, calling it "poison." In fact, the allusions and symbols in the poem were a way for Shelley to call out these critics, whom he blamed for Keats' death.
Throughout the poem, his belief that the youth died too early is apparent. So, too, is his love for Adonais. Because of his reverence for Keats, it's possible that Shelley meant us to see the speaker as himself. After all, Shelley made his reverence for Keats known, both before the youth's death and after (a copy of Keats' poetry was even found in Shelley's pocket after he died). All the same, since he doesn't name any names and instead sticks to allusion and symbolism, we'll just have to keep calling him "the speaker." Perhaps most importantly, what we see clearly in this poem is the speaker working through the several stages of mourning, from shock at the death of the youth to anger at those that did not protect him (and those that may have been responsible) to sorrow and, finally, to acceptance.