If Katniss were a Slytherin, she wouldn’t have volunteered for her sister in the reaping
or wanted to sacrifice herself for Peeta
or risked her life in the rebellion
or risked her life for anyone, really.
she’s a Gryffindor
Throughout the entire course of the story she sacrifices everything for Peeta and Katniss and the well-being of Panem. Even in her weakest moments, she draws strength from the thought of her sister, her family, all of the people who support her.
In the arena, she would’ve had no quelms over killing Peeta if she were a Slytherin. She’d have ignored her feelings and been able to stay focused on winning, but instead she just hoped someone else would take care of him, because she wouldn’t have been able to kill him.
Also, she always felt the need to repay people who did things for her. If she were a Slytherin, she would not only expect people to do things for her, but would feel no guilt over it at all.
Throughout the entire course of the story...sacrifices everything