Thrawn. Like Alan Trammell, or Rich "Tombstone" Jackson, or Sidney Moncrief to their sports' halls of fame, Thrawn is the greatest Star Wars character lurking at the edges of full membership in the canon, through a film role, or even a mention.
Retroactively de-canonized (as an Expanded Universe character) after Disney purchased Lucasfilm in 2012, the Grand Admiral was drawn back in through Disney XD's Star Wars: Rebels animated series, which began in 2014. He returns this spring in Star Wars: Thrawn, a novel set in the time before the events of 1977's Star Wars.
Star Wars: Thrawn is written by Timothy Zahn, who created and gave life to the character (and, effectively, the Expanded Universe) in what is called the Thrawn trilogy of books from 1991-1993, set after the events of 1983's Return of the Jedi.
USA Today has an excerpt of the novel, from chapter 2. The book explores the rise of the Chiss commander — notably the only non-human to hold any significant rank in the Imperial Navy. In the chapter, Thrawn has been discovered by the commanding officer of an Imperial warship who senses potential in him, and taken captive to Coruscant.
In this setting, it's revealed that our blue-skinned, red-eyed art-lover knew Anakin Skywalker before he became Darth Vader because, of course, everyone knows everyone in Star Wars.
Star Wars: Thrawn, published by Del Rey, will be on shelves April 11.