Also, the Tau ace is Commander Farsight, as earlier suggested, pre-secession.
Or, actually, just Farsight as is, third sphere expansion. No one knows why he's there, or how he gets around, but he just pops up, is insanely badass, accomplishes his own strange agenda, utters a line of wisdom, then vanishes again.
To the PC's, he's both a rival and a teacher, through the fights helping them gain the skills to fight the Ethereals.
His long term plan is to have the Imperium be enough of a threat to the Tau for him to subvert the empire, and then using his ultra-militarized empire to strike back, harder than the Imperium, locally, can take.