The story of Beren and Luthien isn't just an old song that Aragorn sings around the campfire in The Lord of the Rings. "The Lay of Luthien," which is the title Tolkien had for it, was a story about one of the most important events in the history of Middle-earth.
Of the three missing Silmarils, stolen by Morgoth and fastened to his crown, one was retrieved not by an army or a powerful Wizard, but by a mortal man and the elf maiden who loved him. There are several versions of this story, not only in Tolkien's musings but throughout the nations of the people of Middle-earth. Some of these early writings are also part of The Book of Lost Tales and The Silmarillion, and a few of the following characters are also named in those pages but in less detail.
6 Dairon
Dairon was Luthien's younger brother in the novel Beren and Lúthien, and he doesn't appear in other versions of the story. He often gets confused with the famous minstrel and bard Daeron, another character who lived earlier in Middle-earth's history and brought letters and music to the people of Beor. He appears in The Silmarillion.
Dairon doesn't appear in other stories, and in the extensive tables laid out for Melian and Thingol's descendants, Luthien is listed as their only child. In the novel, he was playing his pipe for Lúthien as she danced when Beren found them both by chance in the woods. Later, when Beren and Lúthien set out to recover a Silmaril, he tried to follow them but was lost along the way and never found again.
5 Vëannë
Gather around, elf children, and listen to Vëannë the storyteller and The Tale of Tinúviel. Early versions of Beren and Luthien are given this title, and only in these versions is the story imparted to us by this mysterious character that passes on the most essential lore to the next generation.
Before Vëannë begins, she introduces some of the main characters and their own past exploits, most notably Luthien's parents, the king and queen of Doriath. This can be confusing because Vëannë uses names that aren't in the other books, which were written much later. Thingol and Melian are known as Tinwelint and Gwendeling, their kingdom is Artanor, and Morgoth is called Melko.
4 Eriol
The listener of Vëannë's tale, this character could be a stand-in for Tolkien himself. However, given that this character is unfamiliar with the characters in this tale, it could either be a stand-in for the reader, or we're meant to assume that Eriol is very young or still a child.
Eriol asks Vëannë specific questions that make the latter explanation seem more likely. He knows little of the Silmarils and she describes the meeting of King Tinwelint and his Queen Gwendeling as if he's never heard it. The story of Beren and Luthien begins after the introduction of her parents.
3 Tevildo, The Prince Of Cats
Beren and Luthien could not have recovered a Silmaril without the help of Huan, the Wolfhound of Oromë. Such a noble hound could only be opposed by a host of cats, and the Huntsman Thu, who would eventually take the name Sauron, had exactly that guarding his mightly and dark halls. Tevildo, a powerful demon who had taken the form of a large black cat, was the leader of them all.
Tevildo was one of Huan's mortal enemies, and when Luthien appeared on his doorstep with a story about the great wolfhound lying wounded and senseless nearby, he took the bait and went to find him. The clever hound betrayed the cat and tricked him into revealing the magical secrets of Thu's house. This broke the spell on the other cats, returned them to normal size, and took away their powers of speech, but it didn't make them any less evil. It seems that Tolkien was a dog person.
2 Umuiyan, The Doorkeeper
An older cat who seemed to have a more agreeable disposition than his companions, Umuiyan was charged with being the Doorkeeper of Thu's hall and was the first creature to greet Luthien upon her arrival. He agreed to take her to his master, Prince Tevildo, so she could tell him an important message about the wolfhound Huan.
He took her up the stairs but fell under the sleeping spell woven into Luthien's magical cloak. Tevildo assumed that he was failing because of his age and ordered that he be cast into a pit, but not long after that Telvido was defeated by Huan, so maybe Umuiyan's end was not so dire. Miaulë, the kitchen cat and Cook, is another interesting character that worked in Tevildo's service. While Beren was a thrall, he worked in the kitchen under Miaulë's strict and nimble claws.
1 Idril "Silverfoot" Celebrindal
Idril Celebrindal was the daughter of King Turgon of Gondolin and was a direct ancestor of Elrond. She is briefly mentioned in The Silmarillion as the woman the traitor Maeglin loved, but she didn't feel the same way, and the spurned elf took his revenge by betraying her father's hidden city to Morgoth.
Idril's story has more detail than Beren and Luthien other than just her name and family line. As the city was being destroyed by Morgoth's forces, Idril and Tour, along with their infant child, managed to rally a small group of survivors and escape. Beren and Luthien describe in detail how her son Eärendil returned the Silmaril recovered by Beren and Luthien to Valinor, opening the way for her and her husband to sail there at the end of their lives.