A prominent member of the Ghost crew, Sabine Wren spent the early years of her life fighting for the Rebellion in Star Wars Rebels and has been long-rumored to appear in The Mandalorian. Sabine's story is perhaps one of the most interesting ones coming out of Star Wars animation. She was born on the planet Mandalore in 21 BBY, but she never truly saw the planet in its heyday due to the Galactic Empire's formation two years after she was born.
Years later, the Empire occupied Mandalore and built an Imperial Academy there, which Sabine attended and designed a weapon at. Seeing it as a challenge, she constructed a weapon that would later oppress the Mandalorians - her own people - by burning the beskar in their Mandalorian armor. Sabine regretted her actions and destroyed the weapon (or, at least, she thought), along with all of its design plans, and then fled her homeworld. Not much later, she joined up with Hera Syndulla and her Rebel crew.
Sabine was 16 years old when audiences first saw her in Star Wars Rebels season 1, which takes place approximately five years before the events of A New Hope. She continued to fight against the Empire throughout the rest of her teenage years, ultimately acquiring the Darksaber when she was 19-20 years old. At that point, she temporarily left the Ghost to help her family liberate Mandalore from Imperial rule and find someone to accept the Darksaber as the new Mandalorian leader. Sabine was 21 years old when the main story of Star Wars Rebels ended.
In order to avoid any lingering questions about why Star Wars Rebels characters weren't in the original trilogy or haven't been referenced anywhere else, the series ended with a prologue that takes place in 4 ABY (shortly after the Battle of Endor from Return of the Jedi), which explained where everyone was during those years. Sabine's story left off with Ahsoka, as the two were leaving to search for Ezra Bridger. That puts her at 25 years old when fans last saw her on-screen, but there have been rumors for a long time that she will one day appear in The Mandalorian.
Barring any time jumps, The Mandalorian series takes place in 9 ABY - five years after the events of Return of the Jedi and the original trilogy - which means Sabine would be 30 years old. If she were to appear, it would not only be the first time that viewers would see Sabine in live-action but also as an adult. Because of that, she may look considerably different than people realize, disregarding the fact that it would be a jump from animation to live-action which comes with its obvious differences.