To be honest, most of Aragorn's real development as a character as he goes from rough-and-tumble Strider to the leader and king we know he's going to be, actually takes place in the first two books. (See our "Character Analyses" from the learning guides for The Fellowship of the Ring and The Two Towers for more on this.)
By the time we get to The Return of the King, Aragorn has already figured out pretty much everything he needs to do. He has got to go to the Paths of the Dead and find the ghosts who owe his family a favor, and then he has to lead them into battle against Mordor's armies. And then, if he's still alive, he has to take the crown of Gondor, marry Arwen Undómiel, and settle down as High King. It's a pretty straightforward plan, and Aragorn just has to carry it out.
(We'll just tell you right now, our list of what makes Aragorn a good king is going to look a bit like our list of traits that make Frodo a hero in his "Character Analysis." Clearly, Tolkien had some very set ideas about what makes a guy good.)
Here we are, ready for another dose of good old fashioned humbleness, which you will also find in our "Character Analyses" of Frodo and Sam. Aragorn is in a slightly different position from our hobbits, since he is actually heir to the throne of Gondor. He is about to become the most powerful human on all of Middle-earth. What's more, he's the commander of his army: it's his job to give orders and to be certain of them. How exactly is he supposed to be humble under these circumstances?
Basically, once Aragorn arrives in Minas Tirith and takes command of the armies of Gondor, humility means listening to Gandalf's advice and putting Gandalf's plans into action. By now, Aragorn has enough confidence in himself as a leader (versus his worries at the beginning of The Two Towers). But he is also smart enough to know when he needs to shut up and listen
It's this trait that makes Aragorn totally different from Denethor, the former Steward of Gondor. Denethor gets mad at Faramir for listening too much to Gandalf and finally calls Gandalf "Grey Fool" (5.7.34). Denethor is so jealous of Gandalf's influence and so worried about his own position as leader of Gondor that he lashes out at the one person who could most help him in this war against Mordor. Way to be humble, dude.
Aragorn is not as insecure as Denethor. He is able to accept Gandalf's help without worrying that it makes him look weak or not capable as a leader. And that's what makes Aragorn a great king: he is humble enough to accept help when he needs it.
Aragorn marches right up to the Black Gate of Mordor with six thousand men, so that he can knock on Sauron's front door. In many ways, this is an incredibly stupid idea, since Aragorn knows that his troops can't beat Sauron's armies in straightforward battle. He's just asking for Sauron to come out and kick his butt.
But of course, Aragorn's goal isn't to defeat Sauron in battle. He brings his armies there as a distraction from the real attack on Sauron's power, which is happening right in Sauron's backyard: the destruction of the Ring. And that makes this idea incredibly smart.
Unlike characters like Boromir or Denethor, who care mainly about the safety and glory of their own country, Aragorn understands that the Ring Quest is the most important thing in all of Middle-earth right now. His own safety and the safety of Gondor come in second. Aragorn's decision to follow this possibly suicidal, insanely risky strategy is proof that he has perspective on what matters: the Ring.
Aragorn is mostly lucky on this front. As heir to the line of Isildur, Aragorn happens to have inherited a lot of great advantages. But it's to his credit that he is smart enough to put them to good use. For example, the palantír of Orthanc is an heirloom of Isildur's family, which Aragorn uses to confront Sauron long-distance from Isengard. Even the sight of an heir to the royal family of Gondor kind of freaks Sauron out. He knows that Aragorn is going to be a threat to him, and that worry draws his attention to Aragorn and away from Mordor (where Frodo and Sam are still busy questing).
And of course, Aragorn recognizes when the time has come to call in the Sleepless Dead. This is a group of dead guys who promised to help Isildur fight against Sauron the first time around, and then went back on that promise. It turns out that their oaths were binding even after they died. So Aragorn, as Isildur's heir, has the right to order them to help him fight Sauron once again. And with their help, he destroys the boats of the Haradrim, the evil men helping Sauron attack Minas Tirith from the River Andúin.
We have to hand it to Isildur: the Sleepless Dead is one of the creepier inheritances we have ever heard of, but they obviously come in handy.
Okay, we admit it: this list is getting long. But there are so many reasons why Aragorn is going to make an awesome ruler. He's in the title of the novel, after all; he's pretty important to this book. But we'll keep this entry brief. In the Houses of Healing in Minas Tirith, the wise-woman Ioreth reminds Gandalf of the old saying: "The hands of the king are the hands of a healer, and so shall the rightful king be known" (5.8.43).
And of course, we know that Aragorn has the hands of a healer. Remember how he saved Frodo's life with athelas root back in The Fellowship of the Ring? And in The Return of the King, he saves Merry, Faramir, and Éowyn from fading away after being exposed to the Black Breath of the Lord of the Nazgûl.
The symbolism of this idea is interesting, though: why would the hands of the king be particularly good at healing? It's not like Aragorn went to medical school. There seems to be an association of ideas going on: a kingdom without a king (like Gondor, for most of the series) is like a sick body. Nothing works out quite right, and the people go unprotected. But once the king comes back, he will be able to put the body of the kingdom back together, to heal the state itself, so to speak. So, the king's ability to restore his individual subjects' health is a symbol of his ability to renew the kingdom as a whole.
Last one—we promise.
It turns out that everyone adores Aragorn. And that's a good sign for his ability to be king, because you only get to stay king if your subjects are willing to let you rule them. We get proof that Aragorn can inspire loyalty all over this book. After all, Gimli, Legolas, Elladan, Elrohir, and the Dúnedain all agree to walk with Aragorn through the evil, cursed Paths of the Dead just because they love him oh-so much. We can't think of many people we would follow through the land of the dead, so we're impressed, to say the least.
Not only does Aragorn inspire loyalty from his followers, but he also has an incredibly devoted girlfriend waiting to join him in Gondor once the war is over. Arwen Undómiel, Elrond's daughter, gives up immortal life so that she can stay with Aragorn. That's right: she chooses eventual death, just so she can hang out with her hubby for a few years. That's some serious devotion.
It's like a total reversal of the vampire thing where, if you're going to live forever anyway, you might as well change your loved one to join you in eternal life. In this case, Arwen chooses the reverse. She can't change Aragorn, so she'll die when Aragorn dies. Now that's a real sacrifice for love. To be honest, Bella Swan's choices seem a heck of a lot easier than Arwen's.
It's a shame that the love story of Arwen and Aragorn gets stuck in the appendices at the back of The Return of the King (Appendix A, section v, to be exact), because it is pretty romantic, something that The Lord of the Rings doesn't have much of otherwise. With Arwen at Aragorn's side, the loyalty of his subjects, those famous healing hands, his family heritage, and a good sense of both perspective and humbleness, we're pretty confident that Aragorn's going to make a fabulous High King of Gondor.