Who is Grand Admiral Thrawn?

Grand Admiral Thrawn is that blue guy everyone was freaking out about back in July when he made his live-action debut in the official Ahsoka trailer. In Rebels, Thrawn was voiced by Lars Mikkelsen (yes, he’s Mads’ brother, and he also plays Stregobor in The Witcher Netflix series), and he’s reprising the role for the Ahsoka series, much to fans’ delight. Thrawn is an interesting guy—he’s a non-human who reaches an impressively high rank within the Empire, thanks in large part to his tactical prowess and ability to be one step ahead of the Rebels. He also, weirdly, loves art from other cultures (how very colonizer of him), and he’s considered the “Heir to the Empire.” In fact, he first appeared in a 1991 novel of that name by Timothy Zahn, and though that book is no longer canon, Thrawn quickly became a fan favorite, and he was re-introduced into canon continuity via Star Wars Rebels and a new set of books by Zahn in the mid-2010s.
In Rebels, he’s pulled into hyperspace by Jedi Ezra Bridger and disappears, the foundation of the Empire going with him. But, it’s clear from the Ahsoka trailer that those who believed in Thrawn are trying desperately to bring him back to fill the vacuum left by the Empire’s demise.