Fíli comes off as pretty arrogant and flippant—kind of a jerk, actually, if we're being completely honest with ourselves. He struts into Bag End like he owns the place, not listening to what Bilbo's spluttering their way and not stopping to consider that the poor hobbit doesn't know what's going on... He's cocky and headstrong, and it seems as if nothing can faze that front.
But that's an act—at least part of it, anyway. He's not arrogant and he's not uncaring and he's not an ass to the people he loves, but he pretends to be so he can have control over at least part of his life. He does this because his existence is so unsure; he never knows where he is headed, whether he will even live to see the next year's spring. He's a dwarf who grew up in exile with the weight of a lost kingdom on his shoulders, and that burden only ever becomes heavier as he grows because Thorin is king, but he's the crown prince…and that means that he will himself rule one day. He's lived the last eighty-two years of his life preparing for the days when he'll be King under the Mountain, and it haunts his every step that he will soon be charged with the fate of thousands of his brethren.
He knows he's not ready for this. He doesn't think he'll ever be ready for such a heavy title, because he's young and inexperienced and he's never even seen the mountain they're risking everything to reclaim. All he knows of Erebor are the tales his elders told him as a child, by a flickering fire or as he snuggled into bed for the night, begging for just one more story before he fell asleep. Because that's all Erebor has ever really been to him: stories of some bygone age that have never quite been within his grasp. And he was quite content with living like that, allowing his birthright to be some far-off tale of dragons and jewels…right up until Thorin announced his intentions of reclaiming the mountain.
He goes on this quest for his uncle, because he thinks it's the right thing to do, because he wants to prove his worth to his family and to his people and to himself. After all, what is a king who has not earned his subjects' respect? He goes along to prove himself, but deep in his heart, he knows he's nothing—nothing—compared to Thorin. Thorin Oakenshield is a legend, a hero of the Line of Durin whom every small dwarfling idolizes and aspires to be one day…
Fíli's not any of those things, and he knows it…but he doesn't really mind, because if he's completely honest with himself, he thinks he idolizes Thorin too. (Maybe that's a good thing, and maybe it's not, but Thorin is the dashing hero from all his childhood stories so he thinks maybe, just maybe, he can get away with such thoughts.)
So after the battle with Azog on the cliff, when there is a very real chance that Thorin is dead, Fíli panics. He panics because if Thorin is gone then that makes him ruler of a people scattered to the four winds, and that makes him king of a mountain he's never known; he panics because he knows he's nowhere near prepared for this…
But there is also a different panic, a deeper and more hysterical one, that rises above all of that. He panics because that is his uncle, bleeding and battered and far too still, clutched there in the grasp of an enormous eagle. That's his uncle, who became his second father after Dís' husband passed away—his uncle, whom he knows he would go to the ends of the earth for…
He'd do anything for Thorin because he's family, and that's all Fíli's ever had, growing up in poverty and exile and disgrace. All he's ever held dear is his kin, and he's known since childhood that he needs to protect his mother and his little brother at all costs. Family is the most important thing a person can have in this world, after all, and he would be nothing without it.
He's known this…and yet he's never thought he would have to protect Thorin as well. But here, he realizes that his uncle isn't the invincible hero like he is portrayed in all the stories. Thorin is terrifyingly fragile and mortal and no more incredible than any other dwarf, except for the fierceness of his tongue and the stoutness of his heart.
So through the panic and the terror and the pain, Fíli makes his decision. Without his family, Fíli is absolutely nothing, so he knows exactly how far he will go to keep them—all of them—safe. And this is the exact moment when he realizes this, when all the cockiness and youth and visions of grandeur finally fall away for good…and all that's left is Fíli. Not Fíli the crown prince or Fíli, heir of the legendary Thorin Oakenshield, but Fíli the brother. Fíli the son. Fíli the nephew. Fíli the boy.
That's all he's ever been and all he ever will be, and that's the Fíli who will die for everything that's ever mattered to him in just a few months' time. He knows, even now, upon the backs of the great eagles, that he will protect Thorin with his last breath. He will protect Thorin until he physically cannot stand anymore, and his insides are spilling out across his lap, and the blood pumping through his stubborn heart is slowing slowing slowing until it finally gives up his life.
And on the desolate fields before Erebor, that's the Fíli who will stand over his wounded uncle—his wounded king—fighting off impossible waves of orcs until he is finally struck down. He stands back to back with his brother (who also knows he's going to die, but Fíli denies such things to himself...because even if he knows he has to protect Thorin, he's more terrified than he's ever been in his life because Kíli is in danger) until they fall to spear or sword or arrow…until they fall defending the one thing that matters more than all the gold in the mountain.
(Fíli knows, now, that he will never rule Erebor, but it doesn't hurt like he thinks it probably should. But then he supposes that it's always been his fate to protect his family, and he would never forgive himself if he lived while they did not. Perhaps it's not so strange after all.)
Maybe he lies on the battlefield, only able to wait for death to dull the agony of his wounds. Maybe he lives long enough to see Beorn arrive, watches with failing eyes the ever-so-slight rise and fall of his uncle's chest that means that there's still hope and Erebor will yet have its rightful king. Maybe he lives long enough to see the life flicker from his beloved brother's greying eyes. Maybe he lives long enough to see someone he knows—someone he recognizes and loves as family…just not the family he had to protect with nothing but his own, broken body—rushing toward him. Maybe he lives long enough to see the utter horror in this person's gaze as weapons fall from nerveless fingers and bloodied hands grasp his shoulders, begging him to hold on.
Or maybe he doesn't see any of these things, and is dead long before any sort of friendly face arrives to ease his passing. Maybe he is dead even before his mangled body kisses the ground.
And then maybe none of this really matters, in the end, because the story of Fíli has always begun and ended with those closest to him. When his uncle falls under Azog's mace (never to recover, though Fíli does not know this and never will) and his brother succumbs to countless, nameless, vile creatures who only seek to destroy all that is good in this world…that is when Fíli falls as well, even if he still breathes and his body still clings desperately, stupidly, to life.
Because without Thorin's gruff affection washing over him when he does well, without Kíli's barking laughter breaking through even the darkest haze within his mind…without either of these things, Fíli is nothing—absolutely nothing—and even if his body had survived the battle, his soul would have been lost beyond anyone's reach. Those left behind think it a mercy that they died together, that the Line of Durin perished as one, because they had always been three parts of a whole…and every one of the Company has always known that losing one would mean losing all of them.
(Of course, that does nothing to ease their own agony as Glóin and Dori carry in the princes' broken corpses, tears falling unashamedly down their cheeks.)
They are buried together, as they should be. Their tombs are elaborate—nothing less than art carved from stone. It is fitting, some say; after all, Thorin was (should have been) King under the Mountain. He and his nephews, the legends who reclaimed their home from the wrath of the dragon, deserve nothing less.
But those who knew them best know that had they heard such accusations, Thorin would scowl and turn away; Kíli would only laugh; Fíli would scoff and shake his head. Each would deny that he is different from any other dwarf, would say that legend would never describe someone like him...
Everyone knows that's not quite true, though, because even if Dáin now sits upon the throne, he doesn't truly belong there. He is a just king, of course, wise and strong, but he isn't Thorin. Even if the crown rests now upon his brow, and it is his son who stands at his right hand as heir, Dáin Ironfoot is not King under the Mountain, for he did not fight with body and soul to gain it back. In their darkest moments, the remaining members of the Company think he does not deserve the throne at all.
And sometimes, if they're paying too much attention (or not much at all), they can hear echoing peals of laughter from a distant hallway, or see the edge of a long-forgotten fur coat turning a corner, its owner walking with purposeful, long strides. Sometimes, the throne has a different king upon it, and there are two princes at his side instead of one... Sometimes, the world seems to right itself again.
But then the laughter inevitably fades; when they turn the corner, there is nobody there; they blink, and Dáin is again sitting on the throne...
And they are left cold and alone in a world devoid of their sources of laughter, of their source of strength.
(The Line of Durin has fallen, leaving behind a kingdom that should have—could have, would have—been theirs.)
The Line of Durin has fallen, but at least they are together...and even through their pain, those who knew them best realize that they could ask for nothing more.