The Lord of the Rings feels like the most epic modern story in existence, but after a fan has read The Silmarillion, it seems even bigger. The Hobbit has its place in this franchise, but it's barely even a backstory compared to the scale and breadth of the adventures of The Silmarillion. Edited by his son Christopher and published later than the more popular novels, J.R.R. Tolkien's prequel is mostly about the creation of Middle Earth and the early royal houses of Elves and Men. It has nothing to do with the Rings of Power. These were created during the Second Age of Middle Earth, and The Silmarillion takes place during the First Age.
There are several characters that are so essential to the lore of Middle Earth that the War of the Ring could not have taken place without their power and influence, but they only appear as characters in The Silmarillion. Fans of both the LotR books and movies will recognize some of these names, however, as they are often mentioned in songs or poetry, or even invoked as a raging battle cry.
6 Morgoth
Sauron was little more than a gifted minion, as far as the plot of The Silmarillion is concerned, and he appears in both the older stories along with The Lord of the Rings. The main antagonist in The Silmarillion is Morgoth, and although his name is invoked occasionally in the main trilogy, this is the only book in which he appears.
He was known as Melkor before Fëanor renamed him Morgoth, and he was one of the Valar, godlike beings who created Middle Earth. His first name meant, "he who rises in might," but his second was blunter, "Black Foe Of The World." It's safe to say that Fëanor took the theft of the Silamrils personally, and Morgoth lived up to his dark title.
5 Fëanor
The book is called The Silmarillion, and the creator of these wondrous jewels was a master smith named Fëanor, who was also the ancestor of many great and noble Elven houses. He enclosed the light of the Two Trees in these Great Jewels, using means unknown, and when Ungoliant and Melkor destroyed the trees, their light existed nowhere else.
During the same attack, however, Melkor and his spider pet had also killed Fëanor's father and taken the Silmarils. He vowed to pursue the fallen Valar to Middle Earth, and his seven sons took the notorious Oath of Fëanor, in which they vowed to pursue and violently subdue anyone who would take a Silmaril from them. Many of the Elves followed and were caught up in this oath, which had deadly repercussions for generations, including the Kinslaying.
4 Fingolfin
Fingolfin is the reason that Morgoth was weakened enough that two other important characters, also exclusive to The Silmarillion, were able to retrieve one of the gems. The half-brother of Fëanor, Fingolfin was able to deliver one of the first definitive blows against Morgoth, giving him a permanent wound during a one-on-one battle, even though he gave his own life in the attempt.
There was a lot of family drama that involved Fingolfin before the Oath of Fëanor and the exodus of the Eldar to Middle Earth, mainly about Melkor secretly accusing him of trying to usurp the throne held by Fëanor. Later he was an unwitting participant in the first Kinslaying, and survived the fight with the Lord of the Balrogs in which Fëanor was killed.
3 Melian The Maiar
One of the spirits known as the Maiar that were created at the beginning of time, Melian was one of the few that were named. Although she was one of the most powerful, she was not one of the five Maiar chosen to become Wizards. Instead, she seems to be the only one who traveled to Middle Earth of her own free will.
Melian has a role in The Silmarillion and also appears in both volumes of The History Of Middle Earth. She used her magic to protect the Kingdom of Doriath, and the most powerful minions of the Dark Lord were unable to breach it, but she predicted one day that a single traitor would bring the walls down. Melian had a single child, Lúthien, who managed to wrest a lone Simlaril from Morgoth's iron crown.
2 Bëor The Old
One of the first of the race of Men that makes the early history books, Bëor The Old was one of the first human kings to make contact with the Eldar. There were few humans in Middle Earth at the time, which is why they don't have a lot of representation in The Silmarillion. Their role at this time was as little more than vassals for a much more powerful race.
Despite some cultural disadvantages, Bëor was popular among the Elves, and his relationship with them laid the foundation for generations of cooperation between their kingdoms and his own people. Among his descendants are famous names such as Lúthien, Elrond, and Arwen.
1 Beren & Lúthien
These two characters share an entry because that's how they would have wanted it. Aside from Fëanor, these are actually the main characters of The Silmarillion, which is odd because it takes about three-quarters of the book for them to show up. Arwen and Aragorn's union in the main Lord of the Rings story is meant to be a happy echo of this ancient legend.
All that exposition was about all the events that took place in order for Beren and Lúthien's heroics to be possible. Considering how Morgoth and his dark court are built up throughout the story, the way these two take on some simple disguises and get a lucky roll on a Sleep spell sounds more like a Dungeon Master's nightmare than something Tolkien wrote. But that's how they manage to retrieve one.