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.
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.
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.
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.
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.
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.
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."
