Shas'la T'au Kais
Late in the 41st Millennium, the Ethereal Aun'el Ko'vash was kidnapped by Space Marines under the orders of Imperial Governor Severus. The Tau response was swift. Fire Warriors descended upon the world of Dolumar IV with a vengeance, intent on getting their Ethereal back.
One of these Fire Warriors was a young Shas'la by the name of Kais. In his very first battle, his "Trial by Fire," Kais proved instrumental not only in the initial rescue, but also in the resulting battle against the forces of Chaos, who had orchestrated the entire incident in an attempt to release the Daemon Tarkh'ax from a dimensional prison. Guided only by his commander Shas'el T'au Lusha, and later Captain Ardias, Commander of the 3rd Company of the Ultramarines, Kais fought through a hell of blood and fire before finally facing down Tarkh'ax himself.
Reports on the incident are varied, and often contradictory. Some claim that Kais was able to defeat the Daemon singlehandedly, while others state that he was helped by both Captain Ardias and a Crisis Suit team led by El'Lusha.
In the end, Ko'vash was unfortunately killed, and Dolumar IV was duly cleansed by the Imperial Navy. Kais ended up in a mental hospital suffering from a extremely severe case of post-traumatic stress disorder. It is possible that he may recover himself of the Mont'au within, perhaps using the sliver of display wafer given by his father as a guide. The display wafer given to him by his father reads:
No expansion without equilibrium.
No conquest without control.
Pursue success in serenity
And service to the tau'va.
Shas'o T'au Shi'ur
The remaining sliver only reads "With pride." Kais doesn't need the rest because he had memorized it. He crushed the wafer on Dolumar IV and the only reason he has this sliver is because El'Lusha found it while tracking Kais and put it in his hand at the hospital.
Fate After Fire Warrior
Some fans speculated that he later became Shas'o Vior'la Kais, a character mentioned in the Tau codex. This is a mistake: Kais' full name is Shas'la T'au Kais which means he was born on the planet T'au, not Vior'la.
It is also believed that he may be the commander (Shas'o T'au Kais) leading the assault on Kronus in Dawn of War: Dark Crusade, as this Kais is also from T'au. However, "Kais", which means "skillful", is a common name in Tau society (for instance, O'Shovah's full name is "Shas'O Vior'la Shovah Kais Montyr").