It isn't an advisor as per se, what I mean is they get to use a unit from one of the advisors to defend their starting base early, and if you kill them they respawn as though the ai had used a consumable. The number of units and their respawn time is scaled with difficulty, so the 2-3 the easier difficulties get makes little difference, but in higher difficulties even later on in the game the 6-8 free units constantly respawning as you push into the ai can be tricky to deal with, especially as some that some of the different ai commanders/strategies use (this is what determines the unit type) are pretty lategame cavalry units