Most people think the Tolkien library consists of The Hobbit and The Lord of the Rings, with some smaller books on the side that contain glossaries, maps, critical essays, and other reference materials. There's a lot more to the Tolkien library than that, including more than a few novels and stories about the First and Second Ages of Middle-earth, and one of Tolkien's first novels about that bygone time was The Children of Húrin.
Many of the following characters are mentioned or make brief appearances in other periphery works of Tolkien and his son Christopher, such as The Silmarillion, Beren and Luthien, and The Book Of Lost Tales, which tell condensed versions of the tragedy of Húrin and his family. In the novel The Children of Húrin they have more detailed roles to play and seem more like real characters.
Húrin, his wife Morwen, and their children Urwen, Turin, and Nienor also appear in other stories as close allies of the early Elven nations and as proud leaders of Men. Hurin was taken prisoner by Morgoth and refused to submit to the Dark Lord's will, and in bitter revenge, his family was cursed by the enemy to wither and die. Elrond mentions them during his Council in The Fellowship of the Ring, and their story was one of the earliest that Tolkien ever wrote.
6 Hador Goldenhead
The ancestor of Aragorn, the oldest son and heir of Bëor the Old, several of Tolkien's works mention his fall at The Battle of Sudden Flame. Only The Children of Húrin describes any of his exploits or personality beyond this sad end, including Hador's immediate family and his travels with his father.
The whole franchise revolves around the House of Bëor in some way, but Aragorn is not a direct descendent of the titular character, although they are part of the same family. Húrin is one of Hador's grandchildren, and The Children of Húrin is the sad tale of how his family line came to an end. Aragorn is a direct descendent of Húrin's brother, Tuor.
5 Arminas Of The Noldorin
Cirdan the Shipwright is another important character in the history of Middle-earth and plays a much more important role in The Silmarillion. In The Children of Húrin, he sent two heralds to King Thingol and his Queen, Meilan the Maia, about the growing might of Morgoth and the growing shadows in the north and east.
Arminas was the herald that did most of the talking and told briefly of their travels along the coast carrying and delivering messages before heading inland to the Kingdom of Doriath. Gelmir, his curious companion, asked about the identity of the grim man brooding over Thingol's court, which prompted Turin to reveal his identity.
4 Beleg Strongbow Cuthalion
Beleg is mentioned in some other tales, for example as a member of the hunting party that tracked the great wolf Carcharoth in Beren and Luthien. These tales tell little more of him than his tragic and accidental death at Turin's wayward hand.
This was one of the tragedies that Morgoth's curse brought on the children of Hurin, and it was a poor way to die after everything Beleg had already endured. He had always been a close friend and companion to Turin and brought him messages and supplies from the Queen herself. The first time any human had tasted lembas bread was when Melian personally gave some to Beleg to be delivered straight into the hands of the wayward Turin.
3 Nellas Of Doriath
An elf maiden that Turin knew when he was a boy growing up while sheltered in Doriath and her testimony on his behalf as an adult is also mentioned in other stories as an important event in the history of King Thingol's court. Turin was accused of killing one of the king's advisors in the wild, but Nellas witnessed the altercation from a hiding place and confirmed that Turin had acted in self-defense.
In The Children of Hurin, Turin's adolescence and childhood in Doriath are told in more detail and Nellas has a bigger role to play as his childhood companion. He learned to speak Elvish from her, as well as the secrets of the local forests and rivers, both reasons he grew up to be honored among the Elves as a talented ranger and important leader.
2 Mablung Of The Heavy Hand
Another one of Thingol's talented warriors who also fought the werewolf Carcharoth, he was the Captain of the Guard and was sent on some of Thingol's most pressing errands. When Morwen, the wife of Hurin, insisted on leaving the safety of the court to search for her son, King Thingol sent Mablung to guard her. Queen Melian warned Morwen to stay, but she left anyway, and by the time the search party realized that her daughter Nienor had followed them in disguise, it was too late to send her back.
A storm, brought on by an attack of the dragon Glaurung, drove Morwen's horse into the woods. Mablung never saw her again, and although he managed to find Nienor, the dragon had taken away her memory. They were attacked again in the wild and separated, and although Nienor eventually found her brother, it was under the worst possible circumstances.
1 Aerin
A human woman related to the House of Dor-lomin, the family of Hurin's wife Morwen, Aerin was forced into marriage to a local clan chieftain named Brodda. After Hurin was lost, his wife Morwen sent her son to Doriath but stayed to care for her infant daughter.
During that time, the region was sacked by Easterlings, emboldened by Morgoth's recent win against the local kings and other lords. They were afraid to attack Morwen, who was rumored to have elf-magic, and although they left her alone personally, they plundered her home and lands. Aerin used her influence as Brodda's wife to make sure Morwen and her child didn't starve and lived long enough to flee to King Thingol. When Turin returned to look for his mother, she told him what he needed to know, then told him to run for it before burning the house down.