Since Book 1, we've know that Peeta is good at public speaking, in contrast to Katniss…who is not. At all. Mockingjay reminds us of Peeta's talent, but with a dark spin:
Peeta seemed to have been waging a sort of battle in his mind, fighting to get the message out. Why? The ease with which he manipulates words is his greatest talent. Was his difficulty a result of his torture? Something more? Like madness? (10.60)
Words were Peeta's weapons, just like archery was Katniss's strength. That's what they used to survive the Hunger Games and the Quarter Quell. It's also something the Capitol's able to take away by "hijacking" Peeta, replacing all his good memories of Katniss with evil ones. By the time the Capitol's done torturing Peeta, he's lost his ability to speak so beautifully and so fluently. He's also ready to kill Katniss, the person whom he really loves.
At one point, when Peeta is only partially recovered and makes a wisecrack, Katniss is stunned:
Peeta sounded like his old self, the one who could always think of the right thing to say when nobody else could. Ironic, encouraging, a little funny, but not at anyone's expense. (21.65)
The old Peeta is present in the book in fits and starts, in Katniss's memories and in brief flashes of his personality breaking through the hijacking. A lot of the time, though, we're presented with a person who simply isn't acting like the Peeta we've come to know and love.
In the absence of the "real Peeta" we learn more about the original guy. Katniss mourns for "[t]he kindness, the steadiness, the warmth that had an unexpected heat behind it. Outside of Prim, my mother, and Gale, how many people in the world love me unconditionally?" (14.8). The old Peeta was Katniss's family.
With or without his old personality, Peeta is one of the most courageous and determined characters in Mockingjay. At one point, he refuses to let his handcuffs be removed:
"Don't [take off the handcuffs]. They help hold me together. […] When I feel myself slipping, I dig my wrists into them, and the pain helps me focus." (23.9-11)
He's an inspiration to Katniss and to the other characters because he doesn't give up, and he really fights to come back to himself, to distinguish between what is "real" and "not real."
It's hard to watch someone we've identified with and rooted for struggle and suffer so much. Now, when he looks at himself, he's horrified by what is there:
"I didn't know. I've never seen myself like that before. Katniss is right. I'm the monster. I'm the mutt. I'm the one Snow has turned into a weapon!" (21.5)
But Peeta perseveres; he has the courage to look his monstrous self in the face and try to heal. And despite being "turned into a weapon," Peeta is ultimately good, and continues to serve as the trilogy's moral compass.
When asked whether there should be another Games, Peeta is the one who speaks out most fervently against it: "No! […] I vote no, of course! We can't have another Hunger Games!" (26.54). Almost immediately, he reiterates, "This is why we rebelled!" (26.57), and pleads with all the other victors to vote against the possibility of another Games. Peeta stands for his own convictions, which, in some ways, echo the pleas he was coerced into making on behalf of the Capitol. Peeta does want to prevent the loss of innocent life, and he wants freedom for all Panem's citizens. In the second, he is unlike the Capitol; in the first, he is unlike Gale.
Throughout the book, Katniss flip-flops between her love for Peeta and her love for Gale. In the end, she shares her life with Peeta because he's drawn to light and life, and will share that with her. Peeta wants to heal, and he wants Katniss to be able to heal too. He offers her "the dandelion in the spring. The bright yellow that means rebirth instead of destruction. The promise that life can go on, no matter how bad our losses" (27.62). Peeta's the one who encourages Katniss to become a mother, to help literally bring new life into their world. Peeta stands by Katniss and helps her create a memory book to celebrate the lives of all the people who were lost.
In the end, it seems, Peeta does come back to himself. And Katniss's life is all the richer for it.