Who's the speaker of "Eldorado"? What's he like (and we're just assuming that it's a he)? That question is a little tricky because the poem is a narrative, which means it tells a story. While it's pretty easy for somebody to "hide" behind the tales they tell, it's not that easy. We can at least figure out a few things about the guy narrating this poem just based on some of the patterns and obsessions he displays.
The poem is the speaker's story of a knight who spends most of life searching for Eldorado and never finds it. Do you think our speaker is an optimistic guy? Doesn't seem like it. In fact, he seems pretty negative, even pessimistic. The knight doesn't find anything: "No spot of ground / That looked like Eldorado" (11-12). Moreover, when he meets that creepy shadow-dude, the only answer he gets to his question about the location of Eldorado is some cryptic advice about crossing some mountains and going through this valley of death.
The speaker's obsession with shadows, shades, and darkness speaks to the same point. There's the shadow that falls over the knight's heart (sounds like depression to us), then there's that completely unhelpful shadow/ghost the knight meets, and then there's those dangerous mountains and that valley of the shadow (which is a reference to death if we ever saw one!). The point is, we doubt somebody who was up for the Mr. Cheerful award would be constantly talking about all this sinister, unhappy stuff.
So why is our speaker such an unhappy fellow? Why does the whole "Eldorado" thing make him think of death and not, say, huge palaces of gold and the dream of riches beyond compare? Well, perhaps the speaker is an old man who still hasn't found his Eldorado—something he's been searching for his whole life. This might explain the negativity or pessimism of the poem. Perhaps the speaker's been so disappointed by life that he (or she—it's never clear) just can't see the possibility of fulfillment anymore. While we don't have enough in the poem to make a definitive case, the story told here reflects a speaker who is likely beaten down by life. For this guy (or gal), the glass is always half-empty.