On the Characteristics page you can find everything about what your characters can do! Most are beneficial and provide a bonus to Magic, Power, and/orDefense in certain situations or are necessary for Quests.
But some characters, while powerful, can have a negative effect. For example, Gnorl, Ork Sergeant has Undiplomatic. This means you can't use him for a Quest that requires Diplomacy.
More common examples affect Magic, Power and Defense negatively, by a value of 2. Scepticism lowers Magic, Weakness lowers Power and Frailty lowers your total Defense by 2. Set 5, Pirates and Fanatics, introduces these.
An other example is the Sellsword. For example Valerie has this characteristic.
Every time you use her for a Quest (by pressing New Quest) she will lower your Gold by 1. It can even go negative. Set 6, introduces Sellswords.
With the bad comes some serious good. All these characters have very high statistics so they can make it easier to get some extra Power for example, at the cost of Magic or Diplomacy.