I'm quite torn on this, as it's not really a fantasy novel and I think you are maybe looking more for a 'bad-ass nihilist adventurer'

But Eli, the vampire from Let The Right One In, hits a lot of the points you are looking for. If you don't mind how grotesque the book is. And that's a big if.

Eli is a... Difficult character to describe; but I think she hits a lot of what you are looking for other than the 'toughness' vibe I'm getting.

She's not exactly some 'deadly assassin' and her nihilism is either very philosophical and contemplative or a borderline temper tantrum.

Eli can have compassion for others, and her compassion for Oskar is the driving force of the story. Mostly though, she's a nihilistic survivalist.