From the export_descr_ancillaries file, I see the following conditions:

;------------------------------------------
Trigger trigger_runner
WhenToTest PostBattle
Condition not Routs
and WonBattle
and IsGeneral

AcquireAncillary runner chance 8

;------------------------------------------

I can honestly say that I've never acquired an ancillary runner. Am I interpreting this wrong? It seems that there should be (some) chance to acquire one of these characters.

Is this condition fulfilled for when a general fights a battle solo, or for when the general makes the killing/last blow?