Fierceness is an animal characteristic. A fierce animal is an animal which is brutal and which kills by instinct.
A human being who’s fierce is an individual who’s brutal and ruthless with others (see cruelty). The difference between a fierce person and a cruel person is that fierceness is a form of bestial violence which isn’t cautious of what they are doing whereas cruelty is a form of violence where you cause a lot of physical and psychological pain in a more cautious way.
Even though this value is mostly considered negative because of the violence it implies, it can also be positive. For example, you can tell that someone is a fierce friend, which means that they are a very supporting and loving friend who would do anything for you. Being fierce can also mean being strong (see strength), being able to defend ourselves and our loved ones, to resist when facing the most difficult situations and to keep going no matter what.