Katniss changes because she has made President Snow angry because of her act of rebellion and made the Capitol "the laughing stock of Panem". She knew that the Hunger Games had to have a victor so she wanted to commit double suicide so that she could make the capitol punished for the crulity the people of Panem have shown.
Gale is a important minor character because Katniss allready has Gale as a hunting partner and makes her do things differently in the arena because of him.
Other Books I've Read
Hitchhikers Guide to the Galaxy