Narcissa refrains from profanity. There is no need to be using crude words when one sounds far more intelligent eloquently describing the situation. Of course, there are few exceptions. Bellatrix, it seems, can always arouse anger that forces filthy words to slip her tongue. Lucius can arouse something else within her that nevertheless also causes murmured cusses.
Narcissa keeps a journal that she writes in diligently. Be warned, privacy intruders (especially Bellatrix), it’s kept under lock and key as well as a few choice jinxes.
Narcissa, unlike both of her eldest sisters, is not deeply enthralled with politics. She does not really care, letting those around her (her father and her future husband) decide her opinion for her. While she does not believe in the intermingling of muggle and magical blood (and is actually staunchly apposed to interbreeding) she does not feel the need of Bellatrix to run off and pledge her life to killing anyone who disagrees with her. She’s too neat for that. And anyway, aren’t muggles good enough at offing themselves without magical interference?
When Narcissa was a small child she was highly aware of how much she just did not look like her sisters. She did not have the dark features of the sharp bones of the rest of the Black brood. Bellatrix told her that it was because she was a love child between their mother and some mudblood scum. That lie evolved over the years to Narcissa possibly being the child of rape (and Bellatrix’s tastes in teasing grew more evolved and dark). She was not the only one, however, to suggest that her birth was perhaps the product of the friendship old Abraxas Malfoy and Druella Rosier had.
Narcissa Black loves her sisters. Plural. She would do anything for Bellatrix and would defend her to her last breath. She acknowledges that Bellatrix has tendencies that are well, out there. She also understands that her sister’s behavior is cruel and unusual. But that doesn’t matter. Andromeda, formerly Black, receives a different form of love. Narcissa will always care for her sister, though there will always be the deep-seated feeling of anger and betrayal for how Andromeda abandoned her. She does not like to talk about Andromeda; it is too painful and still far too raw. It is easier, at times, to pretend she never existed.