Commander Farsight is a veteran Fire Warrior who leads the Farsight Enclaves, an independant splinter faction of the Tau.
Farsight's name is O'Shovah, and had been a key leader during the Damocles Crusade against the Imperium of Man. An independent thinker who fought aggressivley in close combat, he was already set apart by his peers. Though he studied directly under the Tau's greatest military teacher Puretide, he refused to be implanted with the Puretide Engram when he realized that it removed free will and that the Tau's strategies had not yet been adapted to the enemy's psychic abilities, nor the aggressive fighting style favored by the Orks. The engram also left the commanders braindead when removed.
After the war ended when the Tyranids entered the galaxy, Farsight founded his own enclave when he first encountered Daemons. Due to the stubborn unwillingness of the Ethereals to acknowledge or confront Chaos, as well as his increasing disillusionment with the Ethereals in general, the Farsight Enclaves operated independently. Farsight was declared a renegade by the Tau, with all his honors removed and was to be captured on sight.
At some point Farsight acquired an ancient artifact called the Dawn Blade, a powerful sword that unbeknownst to him extended his lifespan with every enemy he slew. He has far exceeded the meager Tau lifespan and continues to lead the Farsight Enclaves to this day.
Farsight is only mentioned by name, but the Emperor has shown willingness to ally with the Farsight Enclaves, as he finds them the most honorable and sensible group among the Tau and reject the brainwashing of the Ethereal caste (though mostly it's because Farsight fights with a really cool sword).
Farsight and his allies speak exclusively in Japanese and spouts off anime cliches.