Elliot Smith.

“No, I think you’re like James,” said Lupin, “who would have regarded it as the height of dishonor to mistrust his friends.”

James was protective and loved deeply those that were his. He was honorable, he had a very black and white view of the world from what we know of. The king who found nothing more dishonorable than not trusting his closest friends--the ones he became illegal animagi with, the ones he created a secret map with and the same ones that he went gallivanting through the woods under a full moon with.

There will come a ruler Whose brow is laid in thorn Smeared with oil like David's boy

(Yes, this is a reference to Christianity, to Jesus and his brow laid in thorns while put on the cross to die.)

But, I also like to think of it and how this would apply to James. There will come a ruler -- he and Lily are a part of a Prophecy; those who have thrice defied Voldemort, with a summer born child. A Prophecy that they go into hiding for, a Prophecy that could kill them. Yet, it wasn't the Prophecy that killed them, but a betrayal of trust.

The Poet

Sirius Black, the poet.

I feel that this might be contested--maybe a lot of people would think this is the role that Remus would slide into, since he already fits into dark academic aesthetic and fandom portrays him as a voracious reader. (Not to mention the way he talks and how he will weaponize his words)

But I think the Poet is Sirius Black.

There will come a poet Whose weapon is His word He will slay you with His tongue

Born and raised a pureblood to the House of Black, likely slotted to become the next head of his house after his father or grandfather. We see from Walburga's portrait during OoTP that she had a virulent tongue, a trait that Sirius inherited from her. We see it in the train car during their first meeting.

“Where’re you hoping to go, seeing as you’re neither?” interjected Sirius.

Between MWPP, Sirius has always been the one to use his words to cut people the most.

“There’s enough filth on my robes without you touching them,” said Black.

Picking at people's hygiene, cleanliness, looks, abilities, or intellect--nothing is/was too far for Sirius.

“Before Wormtail wets himself from excitement.”

That's how he talks to a friend once he's gotten annoyed? What about his enemies? Or those he considers beneath him? (Which are typically those he doesn't find to be intellectually stimulating to him, as we've seen and a few other people on here have pointed out.)

More than half the time, Sirius' chosen weapon is his words. He is the type of character to point out flaws, weaknesses, things that he knows will hurt the most because he wants to make you bleed.

(And to think, these quotes aren't even some of the worst things he's said, I just couldn't find the quotes that I wanted and didn't want to leaf through my bookshelf.)

The Soldier

I know this will most definitely be an unpopular opinion, but I couldn't decide between Peter or Remus as the soldier. So, I put them both in this section, but I am more partial to Peter.

Remus as the soldier because of his status as a werewolf.

There will come a soldier Who carries a mighty sword

His lycanthropy is his sword, and he is one of Dumbledore's soldiers because of his ability to enter werewolf rings as a spy. It's a heavy burden but one that (as Lupin says) only he can bear.

But that is two of three lines, and he doesn't quite fit the soldier role to me.

Peter does, however.

Sure, he's not a brave soldier, the one that movies are made about--and he doesn't have a strong sense of wanting to fight for the little guy.

But he is still a soldier. The soldier who joined because his friends signed up for a war and he didn't want to be alone. The soldier who thought that maybe, just maybe, he could cruise on throughout the war because he's not particularly useful or strong or politically important in any way. He's just another foot soldier.

Until he is important, and he realizes that he doesn't want to die. He's fresh out of school, he followed his friends into a war because maybe they convinced him (or he convinced himself) that this would be a summer thing or a couple years at most and it'll make for a decent story for his future kids or grandkids when he can say, "Yeah, I fought in the war--on the winning side."

But then he is faced with a choice: living as a turncoat or dying.

Nobody wants to die. Certainly not Peter who is barely in his twenties at this point. He hasn't lived. Didn't have a girlfriend, wasn't married like James was. Did he even want to get married? What if he wanted to travel the world?

We don't know.

So, spying? On his friends, on his comrades, on people who look him in the eye and think they can trust him. And it turns out that Peter is kind of good at it. He's remained undetected for a while. Hell, Sirius thinks its Remus and Remus thinks its Sirius. Nobody ever suspects Peter.

After all, why would they? It's not like he's ever been anything more than a coward, right? James and Sirius' tagalong. Peter couldn't hurt a fly, not when he was the fly.

There will come a soldier Who carries a mighty sword He will tear your city down

First his sword was his secret as a spy.

Then he becomes a Secret Keeper. The Secret Keeper.

A Fidelius Charm was advanced magic. Voldemort could make out with the Potter's living room window and still not know they were on the other side, unless--

Unless someone with the key to the city let him in.