Barriss Offee is a Mirialan Jedi padawan from Mirial, the apprentice of Luminara Unduli, introduced in Attack of the Clones (2002), where she is played by Nalini Krishan and does nothing at all. Everything that matters about her happened six years later, in animation.
The Wrong Jedi
Across the fifth season of The Clone Wars, a bombing at the Jedi Temple is pinned on Ahsoka Tano. The Order expels her and hands her to the Republic courts. In the finale, Anakin Skywalker drags the real culprit into the courtroom: Barriss Offee, Ahsoka's friend, who planted the bomb and framed her deliberately.
Why the speech works
What makes the arc extraordinary is that the show lets her explain herself, at length, without undercutting her. Her confession is not raving. It is a coherent political critique — that the Jedi have become generals, that an order of peacekeepers running a slave army has no business calling itself the light side — and nobody in the room answers it. The Council's response is not rebuttal; it is procedure.
What it does to the Order
Ahsoka walks away from the Jedi at the end of that episode, and she is walking away because Barriss was right about the diagnosis even though she was monstrous about the cure. It is the single sharpest thing the animated series did: it gave the villain of the arc the argument the whole prequel trilogy had been circling, and then let her be a murderer anyway.
Later material
Canon has kept her alive as an Inquisitor-adjacent figure in later stories, but the character's centre of gravity is unchanged: she is the Jedi who said out loud that the war had corrupted the Order, and the Order proved her point by how it handled her.
On the shelf
Nothing in prose is essential. The 2013 arc — Sabotage, The Jedi Who Knew Too Much, To Catch a Jedi, The Wrong Jedi — is the text, and it is four episodes long.