Katniss in Mockingjay is at her most intense. She has transformed from the young girl dedicated to the survival of her immediate family and friends to a girl who has taken on the revolution and now feels responsible for all the people of the Districts, struggling against The Capital.
One word to describe her is traumatized. Unlike many heroes and heroines of young adult dystopian novels, the horrors that Katniss has experienced (not to mention those she's actively participated in) haunt her: just take a look at her "mentally unstable" bracelet. Her constant nightmares and fears that everything she has could be taken away continue through the end of the novel. Even years later, with children of her own, Katniss has not fully psychologically recovered from the traumatic events of her youth and adolescence, and it's unlikely she ever fully will.
Another word that fits Katniss in this novel is ruthless. Katniss has always had moral quandaries and discomfort with killing and hurting others. Those don't go away – she criticizes Gale and Beetee when they make traps for the enemy that prey on human compassion and hurt innocents. Still, when she takes command of her rebel unit after Boggs' death and leads them into the heart of the Capital to kill President Snow, she is leading them down a path that will get most of them killed. She herself shoots an innocent Capital citizen without thinking twice. She even votes that the Capital should send its children to a new Hunger Games.
A final phrase to describe Katniss could be unwilling symbol. In Mockingjay especially, Katniss's role as the Mockingjay and a symbol of hope to the rebel is more at odds with her own desire for personal choices and freedom than ever. She wants to fight and "be useful" but can't because the rebels won't risk her getting hurt or killed. Aside from that, all the propos and staged battles and having a hair and make-up crew again rings a little too much like the Hunger Games themselves. It seems like Katniss is putting on a big, elaborate act again, this time for the rebels instead of the entertainment of the Capital. Serving as a role model is not really a strength of hers (she's not a great actress or really easy to work with), and it's definitely not what she wants to be doing for the cause. This tension between the role she's forced to play by the rebels and the role she wants only grows as the novel progresses.