Caranthir was the fourth-born son of Fëanor and called "the dark," presumably in reference to both his appearance and his mood. However, while he is named "the harshest of the brothers and the most quick to anger" in The Silmarillion, his deeds are not as nefarious as those of some of his other brothers, and he does make some small gains towards unifying the people of Beleriand. Like his brothers, however, his good deeds are undone by his pride and impatience and, of course, his oath.
Caranthir wandered moodily through the streets of Tirion, paying no attention to the others around him. The frown on his face was enough to discourage the street-vendors from approaching him; in the past many of them had been victim to his foul temper. In truth, however, the young elf would much rather cry than shout and was seeking a place where he would not be disturbed.
The bustle of the city faded as he passed deeper into the maze of alleyways that made up much of the city. There, in a hidden courtyard used as little more than a light well for the buildings around it, Caranthir finally allowed his façade of anger dissolve. Through bitter tears he thought back on the events of the morning. Orodreth and his younger brothers had been tormenting him once again, making fun of his dark features. The young sons of Finarfin showed no inclination to grow out of it, as Caranthir had been assured they would, and it was only their cousin’s fear of parental retribution saving them from his wrath.
Having spent his tears, Caranthir sat and waited quietly to be found by the one person who could always make him smile.
"Sorrows" by Joanna Shimmin
Little is known about Caranthir before the Fëanorians arrived in Beleriand. The etymology of his Quenya name Morifinwë Carnistir (the latter resulting in the Sindarin translation Caranthir) literally means "dark Finwë red-faced," making it possible to conclude that his appearance is one source of his epithet "the dark." He is said to have black hair like Finwë and a ruddy complexion like Nerdanel. While all of his brothers are ascribed varying skills, pursuits, and ambitions over the course of The Silmarillion, Caranthir's occupation and talents remain unknown. Furthermore, while his brothers are often closely associated in pairs--Maedhros with Maglor, Celegorm with Curufin, and of course, Amrod with Amras--Caranthir usually stands alone, though one may assume a degree of friendship with his brothers. Celegorm and Curufin leave at one point to go "riding with Caranthir east in Thargelion" and Caranthir flees to the lands of Amrod and Amras following the Battle of Sudden Flame and, with them, maintains a watch atop Amon Ereb. It is also said in a footnote to the essay Of Dwarves and Men (HoMe 12) that Caranthir had a wife, but nothing is known about her, even her name.
Maedhros knew that Curufin would have opted for the mountains as well to mine for metals, which would be of use to his craft, but in this case, he knew Celegorm would convince his brother to live on the rich hunting lands of Himlad. Aye, he knew there would be a long debate between him and Curufin, but he also knew that this little brother would hearken to him.
“What do you say? Shall I give these lands to Curufin, or shall you rule those lands beyond the Gelion?” Maedhros offered once more while he reached for a quill.
Caranthir struggled with his anger. “You are offering me lands instead of rebuking me like a young child as I expected you to do so? You are not making any sense to me.”
“Our father raised you Caranthir, not I. You shall have to live with the consequences of your own words, yet I seek to emulate your skills in a diplomatic position fitting you. Is it aye or nay? In case of nay, Himlad will become your dwelling instead, maybe you can find your peace with Finarfin’s scions.”
Excerpt from Devious Council by Rhapsody the Bard
While clues to his early allegiances and friendships are lacking, his enemies are relatively clear. He was not fond of the House of Finarfin, though reason is never given, and is charged with straining the relationship between the Houses of Fëanor and Finarfin after the arrival of the Noldor in Beleriand. Following his brother Maedhros's rescue from Thangorodrim, a council was held to determine who will lead the Noldor in exile, and there, Caranthir insulted the sons of Finarfin for consulting with Thingol about the Noldorin presence in Beleriand. He was chastised and restrained by Maedhros but his behavior led others of the Noldor to question the influence that the House of Fëanor would have on their plans, and years later, Angrod would remember Caranthir's words, using his cousin's abuse as excuse for his revelation to Thingol about the dark past of the Noldor in Valinor.
"Meeting with the Naugrim" by Noliel.
When the Fëanorians moved into the eastern lands, Caranthir was given the realm of Thargelion, and here, his people climbed the Blue Mountains and had first contact with the Dwarves. Friendship failed to develop between the two peoples--Caranthir and his people were openly disdainful of the Dwarves' appearance and the Dwarves resented it--but they did learn much of craft from each other, and the Noldorin presence in Beleriand allowed the Dwarves to return there, bringing wealth to Caranthir. The Haladin also came to dwell in Caranthir's lands, unbothered by his people, until they were attacked by Orcs and Caranthir saved them from destruction. Seeing their courage, he offered them to dwell in friendship, free in his lands, but Haleth their leader chose to move them west, to rejoin others of her kin.
"Haleth and Caranthir First Meeting" by Kasiopea.
Given this, it is probably no surprise that Ulfang the Black and his sons were welcomed by Caranthir some years later, and it was beneath his banner that they marched to the Battle of Unnumbered Tears, where their treachery proved the undoing of the Noldor.
They say I have no mercy.
They idolize my kinsmen: valiant Findekáno, kind-hearted Findaráto. They see nothing good in me, the Kinslayer.
Only blood on my hands.
In Doriath, I prove them wrong.
In Doriath, I find my brother, his life leaving slowly, painfully, through a wound in the gut. The tendons in his neck are rigid; his hand flutters--weak--in mine. He tries to speak.    He screams instead.
I kneel. I unsheathe my sword and--staring into his beautiful, dark eyes--let it slip across his throat.
His blood washes my hands.
They say I have no mercy.
I am on my knees. I wrap one arm around my stomach, feeling blood soak my clothes, my own blood. My brother finds me thus, bleeding, in agony.
He takes my hand, though my cold fingers hardly feel it.
I open my mouth to speak.
It comes out only as a wordless scream.
He brushes my hair back from my face.
“Fear not, Carnistir.” His voice sounds so far away.
I stare into his eyes, my vision growing hazy.
He has drawn his sword, but I barely comprehend it.
“It will be alright.”
A kiss of steel.
"Mercy," a collaboration by Dawn Felagund (I) and Tárion Anaróre (II).
I am neither a leader, nor a follower.
Being the fourth child has always pulled me to go the way of my older brothers and followed by the younger. I felt powerless to resist the stigma of being one of Fëanor’s sons, kinslayers, the cursed. Our fall from grace never allowing us to become what we should have been: powerful kings and princes of the Noldor.
When Celegorm demanded that we take back the Silmaril, I did not care of the consequences. I wanted some peace from the oath that continues to haunt my days. This deathly seductive consuming jewel has driven us to madness so we cannot even see the obvious. To bloody our hands, yet again, to achieve our own peace...what madness!
Realizing this too late, I survey the gruesome scene around me. Splatters of blood cover me, blood of my own people. Celegorm is dying, Curufin is wounded. Amazingly the Silmaril, glistening clean, powerfully reflects the bloodshed we have caused. I am sickened and horrified to my very core. All fight has left me and I feel a blade slice into my heart.
I, Caranthir the Dark, die knowing that I am not worthy of any peace.
"Reflection of Sin ~ Caranthir's Fate" by Alassante
Caranthir's role in the story ends at the kinslaying at Doriath, where he followed his brothers and was killed alongside Celegorm and Curufin. He ends as a character who attempted to achieve unity with the foreign peoples of Beleriand but was thwarted by the treachery that follows the Noldor, particularly the House of Fëanor.
J.R.R. Tolkien's development of Caranthir's character over the years is remarkable in how little it changed before the published Silmarillion. While events were added over time, very little revision to Caranthir's character occurred once they were in place, which is rather unique given the complex evolutions of the other brothers where their roles in various events as well as their motives were wont to frequently change (with Tolkien usually choosing the more villainous portrayal). From his first appearance as Cranthir the dark in The Book of Lost Tales 2 to his role in the published Silmarillion, while pieces were added to his story, his character underwent almost no revision.
What I did seemed to be a wise course of action back then, another ally in our fight against Morgoth, another small step towards our final victory. But who could have known that in the end the tide would turn against us? Who could have read his genuine thoughts behind the sly speech and humble gestures? And still, still I could not help myself but to entertain the nagging thought of how differently the outcome might have been, had I simply turned him away.
It was probably just another act of defiance, I can see that now. See, I can make things right, too. I am more than the nuisance you have to send far into the East to somehow keep him out of trouble. I can be useful, too.
Ulfang’s people were numerous, and I really thought I could make a difference. But what I did, was to bring the traitor into the equation. And it is hard to live with that. It is unbearable to think about where we might be now, what me might have achieved, had it not been for this. Had I not put trust in one as him when so much was at stake.
Excerpt from Nothing But Dreams by Unsung Heroine.
Two interesting changes did exist, however. In a draft published in HoMe 4, in Christopher Tolkien's commentary on The Quenta, he points out that the Dwarves were originally evil and the Fëanorians "made war upon" them. This was changed to "had converse with" them, which evolved into the aloof but lucrative alliance that Caranthir and the Dwarves later have. Caranthir's allegiance with the treacherous Men of Ulfang's house (there Ulfand) appears in The Earliest Annals of Beleriand (HoMe 4), but in the earliest versions, it is Caranthir who gets to exact retribution for their treachery by slaying Uldor the Accursed, a duty that belongs to Maglor in the published Silmarillion.
The lack of clues in The History of Middle-earth only contributes to the mystery surrounding Caranthir's character. While his brothers' motives are often explicitly plain, his own motivations--indeed, much about his way of life--remains unknown to us. He attempts to form alliances with some even as he destroys alliances with others, and it is difficult to tell if, truly, he deserves his reputation as "the dark."