A/N: I've written some Fili & Kili brotherly feels, so why not pull Frerin and Thorin into the mix? This is angsty, as per usual.
"I belong with my brother."
The words hit Thorin with the might of a hammer. He had thought that Fili—his heir, to be sure, but also the peacemaker, the faithful follower—would not dare to defy his direct order. But the squared jaw and relentless turn of his nephew's shoulders had not allowed him time for further argument.
So he had departed, telling himself that such flagrant disregard for the wishes of uncle and king would be reprimanded later, and that expedience alone had led him to permit the lads to go together without further dispute.
Yet as the barge sailed out of the city gates, slipping past the cacophony of cheers and preemptive adulation, Thorin's mind turned to a memory long repressed, a memory whose sun had risen on an icy winter morning long ago in Erebor when he had first learned the word brother…whose sun had set upon the fields of Azanulbizar, and upon his heart.
He turned it over in his mind unwillingly, with the lines of his jaw locked hard, with eyes fixed on the Mountain that he did not see for once. He could see now another blonde-haired prince, whose brown eyes were fervent and childlike, whose gentleness suggested Fili and whose stubbornness was passed on to Kili—a lad who had, too, wanted nothing more than to be with his brother on the day of his first battle.
But the duties of the eldest prince had forbade it then, as always—fealty over family, faithfulness to king and cause superseding the instincts which would have kept Frerin close by him, protected by the skill and practice of an elder brother's sword.
In the end, his king and his brother had fallen.
It was his grandfather whom he had avenged, but his younger brother whom he had held in his arms in Frerin's last moments of wakefulness, before darkness fell upon him (upon all of them).
I belong with my brother.
He would not—could not—admit it to anyone, could not even breathe the thought aloud, but he knew without knowing why Fili had stayed, and why he had been right to do so.
Because perhaps if Thorin had been at his brother's side on that fateful day, his brother would still be beside him.