The narrator of Paradise Lost is an omniscient third person. This means that the narrator is not a character in the story (like Satan or Adam or Eve), but rather an external observer that can enter the thoughts of all of the characters in the story. Milton does this on numerous occasions, often telling us what Satan is thinking about, or what Adam is really feeling. Because he is not a character in the story, our narrator can be in several places at once. For example, in Book 9, he tells us what Eve is doing, but then he shifts and tells us what Adam is doing. In a sense, the narrator is like a puppeteer. He knows the whole story, and he knows how he wants to present it, so he sits back and feeds his readers information as he sees fit.
At many points in the poem it becomes clear that John Milton, the poet, is our omniscient third person narrator. Several times throughout the poem, he interjects, wishing that things could have turned out differently. He even refers to his blindness (beginning of Book 3) and English politics (beginning of Book 9). Not to mention, he often inserts references to his own poem and its relationship to previous literature (especially in Book 1). For all intents and purposes, we can say that our narrator is John Milton, the blind guy who lived in the 1600s, only he doesn't always like to talk about himself, so it's easy to forget.