One of JKR's most beloved characters, Sirius Black, is also foreshadowed in an earlier book. In answer to Dumbledore's question as to where he got the motorbike, Hagrid answers:
“Borrowed it, Professor Dumbledore, sir,” said the giant, climbing carefully off the motorcycle as he spoke. “Young Sirius Black lent it to me. I’ve got him, sir.” (p. 14, SS)
Notice that JKR plants Sirius into the very first chapter of her first book, even though he doesn't appear as a true character until two books later with Prisoner of Azkaban. She also gives a hint as to his character and role with that motorbike and his presence at Godric's Hollow.
In Prisoner of Azkaban, JKR gives Cedric Diggory a cameo, even though his starring role is not until the next book, Goblet of Fire.
"Diggory got the Snitch," said George. "Just after you fell. He didn't realize what had happened. When he looked back and saw you on the ground, he tried to call it off. Wanted a rematch. But they won fair and square...even Wood admits it." (p. 180, PoA)
If even Wood admits it was fair, but Cedric was willing to forego a win for a rematch, then he must be a true champion. It's a wonderful, although brief, introduction to the young man whose sense of honor and fair play makes him a winner with Harry in the maze, and then lose his life because of it.
Layering characters into stories in this manner reveals the depths to which JKR plots her series. To a reader, a series feels more real and connected when an important character doesn't just begin and end their career in one book. Readers love a sense of interconnectedness and a well thought-out plot.
If you're writing a series, plot out each book, its plots and characters, far enough in advance, that you can bring onstage characters from a later story and introduce them to your audience before their closeup.
What have you foreshadowed in your story or series? Have you brought on a character even before they had a starring role?