I have two possibles. One is Pug/Milamber from the series that starts with Magician; I have never before seen such a good depiction of a really powerful wizard in a BAD mood. And the other is Morgaine, from the Chronicles given her name; unswerving devotion to duty, and a willingness to do some really unpleasant stuff to save the multiverse, and carries probably the nastiest melee weapon in fiction. Changeling is seriously powerful, and hurts the wielder every second it's drawn; and she's willing to take that for the cause, too.
While writing all that, I thought of five more - they come as a package, and any of them are people who you wouldn't want to be sharing the same planet with if they have reason not to like you. The five? Christopher, Kathryn, Karen, Constance and Camilla Kinnison - the Children of the Lens.