When Finnick Odair and Katniss first met Katniss did not like him very much. She did not want to be his ally in the games because she did not trust him. The reason she kept him around was because Peeta wanted him to stay and he was a useful ally to have. He aligned himself with Katniss so he could protect her. He needed her alive because she was the symbol of the rebellion that he was helping to start. If Katniss had died then everything they were trying to succeed with the rebellion would have been for nothing. He had also revived Peeta while in the arena because he knew if Peeta had died Katniss would have given up. This had confused Katniss as to why he did it but he could explain why because everyone in the Capitol and all of the districts were watching them.