General's body guard?

This is the problem I wnat to ask you guys:how can I upgrade my general's body guard?I see in custom,historical and quick battle that the general unit is much more powerful and different from the one in campaign.For example, Roman generals in campaign is low-tech units:Equites.But as I can see in custom battle,they are strong Praetorians cav with higher stats.In battle,you can pause the game and double-click the general to see the detail of his unit.An other situation:Egyptian generals are chariot archers and in non-campaign battle they are a kind of nile cav.Or,Pontic campaign generals are cavalry aux,very easy to be crushed by other cav;in custom they change completely:become heavy armoured cataphracts.
So, in campaign can I improve my general units as above to enhance them? If yes,how? Thanks in advance.