@ff03k64 There are some things you can do to influence the AI, especially if you don't want a human player.
If you use the Easy AI, the AI will move towards the nearest enemy-held capital, without consideration of counterattack.
If you put a capital on the map, the AI will give it more importance, even if losing it has no effect on the owner.
You can give the player specific units through events. If you want a sea campaign, just give it ships
You can deny it specific units or limit the number of units it builds of a specific type. If you want a land campaign, don't let it build ships.
You can confine it to specific territories
If you don't want it to anything except be an ally, make it a pro-neutral or don't give it a movement phase.