Johanna is the living, breathing definition of a loose cannon. You never know what she's going to do, and you're never sure if you should trust her. At least she's entertaining. Like the way she strips her clothes off in the Training Center and struts around naked. Yeah, that takes chutzpah.
Like all of the other Hunger Games victors we get to know, Johanna's had a rough time. She makes a point of emphasizing that she doesn't have anyone she loves, so the jabberjays that speak with the voices of family and friends can't hurt her. She implies that she's completely cut off from that sort of human contact. Usually that would be considered a sad thing; only in the arena would it turn out to be a bonus.
During the Quarter Quell, Katniss finally gains respect for Johanna:
"[Johanna] throws back her head and shouts, 'Whole country in rebellion? Wouldn't want anything like that!' [...] No one, ever, says anything like this in the Games. [...] I have heard her and can never think about her again in the same way. She'll never win any awards for kindness, but she certainly is gutsy. Or crazy." (24.42-43)
Johanna isn't safe. She likes to push people's buttons – both Katniss' and the Capitol's. As Katniss puts it, it's hard to know if Johanna is "gutsy" or "crazy."
Up until almost the end of the book, Katniss is mad that Johanna betrayed her and tried to kill her in the arena. She sees it like this: Johanna knocked her out, cut her up, and left her for dead. It's not until the very end that Katniss finds out that Johanna was working to save her life. She did knock her out and cut her up, but she was removing Katniss' tracker so no one could follow her after the escape from the arena. Johanna may or may not have liked Katniss, but she was working to save her life regardless.