Cortana -- she has changed so much over all the Halo games. Master Chief would not be where he is without her. She helps Chief figure out everything that he needs to do. She is his brains and the force that drives him, and she helps him make the right choices.
Cortana pushes Chief to be the best that he can be, and when he doubts himself she tells him that he is doing the right thing . When she “dies” in Halo 4, it tears Chief apart because she was such a big part of him and his life; even if she is just AI.