Let's start out by confusing you. This character isn't really named Ely; that's just the name he gives to The Man. Later he admits it's not his name at all. But since we never find out what his real name is, we're going to call him Ely anyway.
In a book full of nameless characters, Ely stands out. He shares his name with a biblical prophet (Eli) from the Book of Samuel. And he certainly looks and acts part: he's old (how is he still alive?) and hobbles down the road. He speaks in riddles. Some of what he says seems wise and some of it seems a little crazy.
Ely fits in nicely with McCarthy's Revelations-inspired setting (the Book of Revelation is the part of the Bible that tells of the apocalypse). But Ely doesn't say things you might expect a prophet to say. Don't these bearded old men usually proclaim God's anger and the waywardness of men? Not Ely. He proclaims God's nonexistence and says that Death will eventually disappear along with all human life. He's an anti-prophet, really.
It's worth comparing some of Ely's statements with some of The Man's thoughts. Is Ely a real prophet compared to The Man? Or does The Man actually have a bit of the prophecy in him as well? We're not sure – both characters say slightly goofy things, but then they blow your mind in the next sentence. We guess that's how prophecy works.
It's also worth noting that through Ely, we see The Boy's kindness and generosity. The Boy lavishes their supplies on the old man, who isn't even grateful.