Spoilers ahead from Star Wars: The Rise of Skywalker.
Star Wars icon Mark Hamill will always be most famous as Luke Skywalker, but he has played several other characters across the Star Wars galaxy. He added another voice role to close out the Skywalker Saga in Star Wars: The Rise of Skywalker. Before showing up as Luke, Hamill voices a character named Boolio.
Boolio is the guy who gives Finn and R2 a valuable datafile from a traitor in the First Order. When Finn asks how they can thank Boolio, he replies "Win the war." Boolio's unfortunate end is shown shortly afterward, with Kylo Ren presenting his head to the Supreme Council, revealing they have a spy in their ranks. The spy for the Resistance is later revealed to be General Hux.
Some Star Wars fans said they recognized Mark Hamill's voice as Boolio right away, while others (like myself) didn't know until later.
Boolio was actually revealed as a character several months ago as part of the new Lego Millennium Falcon set. Babu Frik was also revealed around the same time, along with new characters like Zorri Bliss or Jannah. That's how a lot of characters are introduced before movies, and that's why it's so special that Lucasfilm decided to keep The Mandalorian's Baby Yoda a secret. No toys. Lost money. But great surprise.
Boolio marks Mark Hamill's fourth Star Wars character after Luke Skywalker, the voice of Darth Bane in Star Wars: The Clone Wars, and Dobbu Scay in Star Wars: The Last Jedi. Dobbu was the little guy in the casino who confused BB-8 with a slot machine.
Mark Hamill previously spoke out to say it had been mistakenly reported that he only did the voice work for Dobbu Scay; he said he filmed his Star Wars: The Last Jedi scene in a full motion-capture suit with a giant medicine ball to make him the same size as BB-8. He said he didn't expect to be credited. When he does get credited for such things, he likes to use a pseudonym that combines his brothers' names -- Patrick Williams.
At this point, it looks like Mark Hamill just voiced Boolio, but if Hamill wants to come out and say he did more with it or share other details, I'd love to hear it. Here's more on Boolio from the Star Wars wiki.