In The Hunger Games, how does Peeta stay true to himself during the Games?

Peeta is definitely a character who remains true to his
own principals and beliefs, even in the dangerous and life-threatening environment of
the arena and the Hunger Games, where children are forced into life-and-death
situations. One way that we can see this is the way that Peeta accidentally killed the
girl known as "fox-face" by leaving berries out that were actually poisonous. Note his
reaction to having killed her:

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Not on purpose. Doesn't seem fair somehow. I
mean, we would have both been dead, too, if she hadn't eaten the berries
first.

Peeta is a character
who therefore remains true to his principals and to his emotions throughout the Games.
Even at the beginning, when he makes a rather tenuous treaty with the Career Tributes,
he only does this so that he can be in a position to protect Katniss and help her
survive, which he does at the cost of his own leg.