Characters | General tips

Each army should be accompanied by an appropriate agent. I suggest making a regular use of two of them: the spy and the champion. Your spy will be moving ahead of the main strike forces, revealing enemy troops and using his skills to weaken the strongest enemy army. I recommend to develop his Cunning ability tree, especially: Poisoning and Furtiveness. It is also good to invest a couple of points into an Agent (authority), who may improve the spy's critical chance of every successful action, or into Infiltration, which proves to be extremely useful near the end of the game, when you are swarmed by enemy agents.

You should unlock all the agents as soon as possible - experienced agents are useful in late game.

A champion should focus on zeal. I suggest developing: Militancy (passively increases units experience), Strength (helpful at eliminating enemy agents) and the remaining several points devote to Authority (so to increase the effectiveness of actions). This character should be best matched with an infantry- or siege-focused army.

Leader

The last type of agent is a leader. Because of a wide variety of his roles, it is appropriate to devote some more attention to him. Leader's abilities should be adjusted to the type of army he commands as well as the role he plays. Leaders with passive abilities which increase public order, speed up province development or improve recruitment should focus on authority attribute. Skills like General or Command can allow them to recruit more experienced warriors. Such a character should be stationed within your borders in order to train new troops and improve province development.

Another category consists of leaders who care for logistics. These characters should use Cunning, and especially the Tactician ability, which increases their movement speed. Along with extra movement range on the campaign map, the Strategist ability makes the leader excellent at commanding siege-focused armies. Add Siegecraft and Mercenary Contacts to it and you will get a source of cheap cannon fodder, which can be thrown at enemy fortifications in the first wave of attack.

Zeal is best used for cavalry commander. Soldier, Offence, and Fearless Warrior increase effectiveness of cavalry charge, while Dread helps to quickly break through the frontline. This character plays a key role in attacks at well-equipped elite enemy troops.

A question may arise: what should the leader of your faction specialize in? Since this character is immortal (when he falls in battle, you are able to bring him back to game after several turns) I recommend to develop him toward melee combat and put him at the head of a cavalry-focused army, i.e. develop his Zeal skill tree. This way he will fully use all his abilities and in case of a defeat, you will not lose an experienced general.