Red-Haired Shanks is one of the most important characters in One Piece: besides being one of the Yonkou (Four Emperors) who rule over the pirates of the New World, his meeting with Luffy was fundamental to start the adventure of the protagonist of the best-selling manga. Despite that, very little is known about Shanks and his past, but some One Piece's fans came up with a theory that could explain many of the mysteries around this character.
Shanks appeared in the very first chapter of One Piece, as he and his crew were passing through Foosha Village in the East Blue sea. There, Shanks met a kid named Luffy, who wanted to become a pirate. After saving Luffy's life, losing an arm in the process, Shanks becomes the kid's idol and role model, even leaving to him his signature straw hat as a sign of their bond. After that, Shanks would become a famous and powerful pirate in the New World, being recognized as one of the Four Emperors years before Luffy set out to sea. He made brief appearances several other times, never meeting Luffy and his crew directly. It was also revealed that Shanks was once part of the crew of the Pirate King, Gol D. Roger, together with his rival/friend Buggy the Clown. It was never explained, however, how Shanks ended up on the ship of the most important pirate in the world.
According to official information, Shanks is 39 years old. 38 years before current events in the manga, a very important battle took place, called "The God Valley Incident". This was a clash between powerful forces. On one side stood the Rocks Pirates, a crew that comprised many individuals who would later become renown as the strongest of their age, including three future Emperors: Big Mom, Kaido, and Whitebeard. Against them, an unlikely alliance was formed between the Roger Pirates and the Marine, represented by Monkey D. Garp, Luffy's grandfather. The exact circumstances of the battle are unclear, but what is known is that a group of World Nobles (aka the Celestial Dragons) with some of their slaves, was present on the island. The Rocks Pirates planned to attack them because their captain, Rocks D. Xebec, wanted to upset the world order and become the King of the World. The Rocks Pirates were defeated and the World Nobles were able to flee, but this theory posits that one of them left a child behind that was taken over by Roger: Shanks.
There are a few reasons to believe this. First of all, in Kozuki Oden's flashback of his journey on Roger's ship, the Pirate King, while holding Oden's newborn baby, mentions that it's been a while since it's done that. This could mean nothing, obviously, but there is another, much more relevant clue. In chapter #907, Shanks appears in Marijoa in front of the Five Elders, the highest authorities of the World Government (besides the secret King of the World, Im), requesting to talk about "a certain pirate". Marijoa is the most sacred ground of the World Government, it should be an impenetrable fortress, and the Five Elders are the highest authorities of an organization that wants to eradicate all pirates. Not only is Shanks - one of the Four Emperors - not stopped in any way, one of the Elders also says: "We only made time to talk because it was you." This would make sense only if Shanks was a Celestial Dragon by lineage, hence receiving a privileged treatment that would be unthinkable for any other pirate.
Shanks will be the protagonist of One Piece's upcoming film, Red, where hopefully more details of his past will be disclosed, so fans of the manga don't have to wait much longer to find out if there is any truth behind the theory that Shanks is a Celestial Dragon.