Each computer player has traits that define their gameplay aspects, diplomacy, military improvements, civilizations etc. It does not determine their building plan, as it is scripted for every opponent to be the same layout (but in different speeds).
There are two types of traits: Primary and Characteristic Traits
Primary Traits are traits that define their basic gameplay types. It is also determined by the difficulty level, as novice players are always easy, while (most) aggressive ones are in the hard difficulty.
Novice: These computer players struggle building their improvements and their settlements can revolt for a short time (mostly in the beginning). They also do not demand any tribute (except for an alliance), defend their cities against spies, commit espionage and (mostly) do not advance to the Noblemen level.
Balanced: These computer players are not defined by a special trait and thus are defined at their characteristics. That doesn't mean however, that they are automatically weak. Their settlements are balanced and they have a healty amount of military and trade ships.
Merchants: These computer players are willing to trade with you and have strong emphasis to expand this further. They settle many islands and tend to produce more production chains than needed. Their islands often have a Bailiwick and they have more trade ships than military ships (also they tend to have exclusive goods in their ships, should the ship be destroyed).
Aggressive: These computer players are more difficult to befriend and they have a higher tendency to declare war to players they don't like. Their cities are highly secured and they tend to have more military ships than usual. It does not mean however, that they will conquer islands (this is determined by the characteristics).
Characteristics are traits that are more unique for the definition of computer players. However again, the difficulty also plays a role in it (as Willem is more towards peaceful encounters, the Baronessa does not hesitate to use espionage and buy out islands through the city council).
Humanitarian: These computer players like you more if you fulfill all needs to the fullest, collect low taxes and if you complete quests for your civilization. They tend to build bigger cities and their civilization is at most times more happy than usual.
Hildegard von Lewenstein and Helena Flores have this trait.
Diplomat: These computer players are devout diplomats, they tend to consider using more diplomatic actions. A Diplomat is also harder to persuade (this is not shown at the diplomacy screen) and their oriental settlements (if any) tend to have faster and more Envoys.
Willem van der Mark and Baronessa Constanza Zanchi have this trait.
Religious-Oriented: These computer players have faith in their religion. They focus more on faith than other players and settle to the island where Benedicta is (if she is available). However, there are more major differences between the game styles.
Hildegard von Lewenstein, Marie d'Artois and Cardinal Lucius have this trait.
Warmonger: Warmongers are aggressive and tend to declare war against any player. They tend to conquer all islands and destroy factions if strong enough.
Cardinal Lucius, Marie d'Artois (and formerly the Baronessa) have this trait.
Italian: More a cosmetic trait than gameplay-wise, Italians are the only ones that build, buy or request italian Tradesman's Cogs. This is the only trait that was added by the Venice addon.
The Baronessa and Giovanni di Mercante have this trait.
Chivalrous Adventurer: This trait is unique to Sir Gavin Langton. He declares war against you if you declare war against a female computer player. He also tends to buy more occidental drinks. A hidden prerequisite is also that at least a male (himself) and a female player must be active to receive some quests from him.
Black Arts: This trait is unique to Guy Forcas. He tends to fake a diplomatic relation and has a hidden value of the reputation. If he is stronger and the hidden reputation is low enough, there is a chance that he declares war against the player, no matter how high the 'reputation' is (exception is at an alliance). He also tends to use often espionage.
Unstable: This trait is unique to Marie d'Artois. 'Blinded' by the gods, Marie sees a 'vision' and follows "god's" faith to make some surprising actions. There are values that determine what vision she sees, but I haven't figured any more values than the trade pact. If you trade with her often, she will see a positive vision. She also tends to make a lot of random actions. She also hates the Orient, but this doesn't fit in the trait very well.