ACQUIRING THEM.
Defeating a boss or a certain enemy will have a low chance to acquire them.
Ex.
Positive traits
Bravery - adds movement speed, attack speed and ap, decreases dp?
Confidence - adds movement and attack speed, ap.

Temporary positive traits.
Ex.
Tossing a coin to a Wit - fountain... It's done.
Happy buyer (if you buy a certain amount of item to any NPC) - adds silver drops.

Dying otherwise or if an enemy reduce your hp to a certain level will cause you to acquire a low chance of negative traits.
Ex.
Negative traits
Cowardice - minus something to attack speed, movement speed and ap, dp.
Amnesia - lowers xp acquirement. XD
Weakness - lowers ap, dp.
Sickly - lowers dp, movement speed.

Temporary negative traits
Ex.
Broken bones (acquired through enemy lowering your hp to a certain level - low to mid chance; abusing weight limit has a higher chance.) - lowers movement and attack speed, ap.

A certain scholar can add or remove them.

Acquiring a similar trait will level it up. A book sold by the scholar can level up the... some traits to a certain level.

Removing them requires time. A book sold by the scholar can remove them. Defeating the same type of monster that place that negative traits (some traits, like cowardice) will have a low chance to remove the negative trait/s.

Other traits?
Social traits? Adds bonus items or etc.
Ex.
Grumpy (Acquired if you upset NPCs to a certain level?) - lowers bonuses in quests, etc.
Charming (if you friend a number of NPCs) - add bonus items, silvers, xp, etc.