La'Kais is a Tau warrior.
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. While only on his 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 El'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 cleansed by the Imperial Navy. Kais' fate afterwards is also unknown, and the subject of controversy. Some claimed he returned to his duties and finally become a Fire Warrior Commander, while other reports place him in a hospital on T'au, recovering from severe physical, mental, and possibly spiritual trauma. At the end of the book Fire Warrior Kais is battling psychological insanity most likely from the Chaos pit he had been in at the end of the book. So it is possible that he went on to serve.
It should be noted that the usual basis for the former claim, a small story in the Tau codex mentioning a Vior'la Kais, is not necessarily referring to the same Kais, as that Kais is from the Vior'La sept, and La'Kais of Fire Warrior is, according to the novelisation, from T'au (which would make him Shas'La T'au Kais). In addition, Kais is probably a fairly common name amongst Fire Warriors and probably even amongst Tau of other castes.
In the expansion of the PC game Dawn of War, Dawn of War - Dark Crusade, the Tau Commander of the T'au Army sent to Kronus is named Sha's O'Kais, creating the possiblity of the commander being the same Kais of Fire Warrior.