Johanna is one of the few surviving victors from the Quarter Quell. The rebels aren't able to rescue her at first, and she's left behind to be tortured in the Capitol, along with Peeta: "That's how they tortured her in the Capitol. Soaked her and then used electric shocks" (18.33). Though the rebels rescue her, Johanna comes away with a fear of water. As much as she wants to return to the fight alongside Katniss, the torture has damaged her too much: Johanna simply isn't combat-ready, even after she overcomes her morphling addiction – water is simply too hard to avoid.
Like other veterans of the Games, Johanna doesn't have a lot left to live for: she's lost all the people she cares about. While this loss protected her somewhat during the Quarter Quell, it gives her deep sadness when she's been evacuated to District 13 and is left without anything – not even a small object or token, to call her own.
Towards the end of the book, Johanna shows her lust for vengeance by voting for another Hunger Games, to punish citizens of the Capitol: "Why not? […] It seems very fair to me. Snow even has a granddaughter. I vote yes" (26.55). She wants things to be "fair." She wants to see justice done for the terrible things she endured. Johanna seems to be saying that if she had to suffer through the Hunger Games (twice), so should the relatives of people who forced her to (like Snow).