Luminara Unduli is a green-skinned Mirialan Jedi Master from Mirial, introduced in Attack of the Clones (2002) and played by Mary Oyaya. She commanded the 41st Elite Corps in the Clone Wars, fought at Geonosis and Kashyyyk, and trained Barriss Offee — Mirialan Jedi traditionally taking Mirialan padawans.
She is written as the Order's discipline personified: precise, unhurried, faintly severe, the master everyone assumes will produce the model apprentice. That her padawan becomes the Order's most articulate internal critic is the kind of irony The Clone Wars was good at and mostly left unstated.
Her fate is one of the nastiest in canon. She did not die in Order 66. The Empire took her alive, executed her later, and — as StarWars.com's databank entry states — used holographic transmissions of her captivity as bait to draw surviving Jedi out of hiding. Star Wars Rebels plays that out on screen when Kanan Jarrus and Ezra Bridger walk into the trap and find her remains in the cell.
It is a small, cruel piece of worldbuilding that says more about the Empire's methods than any number of Star Destroyers: the corpse of a Jedi Master, kept on file, still working.