At this point, Star Wars is fairly infamous for the fact that even the most minor of background characters has had a story dedicated to their exploits. Some may bemoan that aspect of the galaxy far, far away, but to myself and countless other fans, knowing that TK-421 or "random mouse droid #4" actually had a history before they got ambushed in the Millennium Falcon is actually kind of cool.
The fact that so many characters have found renewal in the expanded universe (whether Legends or canon) is also a real testament to the strength of the novels, comics, TV series and video games that comprise it. Often times these stories have been able to take background figures and provide them with their own compelling arc, which can then lead to viewers reappraising a given sequence in the films.
That's not always the case, but when it is the results aren't to be underestimated. Most Star Wars fans will agree that both Clone Wars series managed to elevate the Prequels, while other books, games and comics have been busy enhancing both those films and the Original Trilogy for years.
Not every character is able to be a game changer, but they're rarely ever not interesting.
10. Willrow Hood
Willrow Hood was - and still is - a bit of a meme among Star Wars fans. The character can be spotted once Lando Calrissian sends out the evacuation order across Cloud City near the end of The Empire Strikes Back, panicked and running with an ice cream maker (although it was obviously meant to be something else in Star Wars terms).
Because of the fact the prop was so obviously an ice cream maker, fans latched on to the extra and they were eventually named in the Star Wars Customisable Card Game, which released trading cards from 1995-2001. Hood was an old Tibana gas miner in Legends continuity, but was given a slightly different origin in the current continuity, revealed in the "Due on Batuu" short story from the second From a Certain Point of View Book released in 2020.
Now, hood was a disgruntled engineer who fixed pressure leaks, with dreams of making it to the big leagues as an adventurous smuggler. He eventually gets his wish, being paid a handsome sum to deliver a camtono (the ice cream maker from Episode V) to its intended recipient on the planet Batuu.
The story makes clear how Hood was in way over his head, and after almost losing the camtono in the confusion sparked by the evacuation (and a double cross from his hired Cloud Car pilot), the package gets destroyed anyway.