Rage
“Pain is just rage I have not spent yet.”
Every hit she takes feeds her. That is not a metaphor for a warrior, it is how the class works, and Harper took it to heart long before anyone explained it to her. Getting hurt does not slow her down. It gives her more to spend.
She does not hide the anger and she does not apologise for it. She aims it. Every point of rage goes into the thing that is threatening her group, and none of it goes anywhere else.
In practice: never sit on rage, Mortal Strike on cooldown, and save enough for Overpower when it lights up.