The master chief who always wears the master chief costume in Halo 4 also called John. His personality is further explored in the books than games. This is so that the player easily projects their own personality on the master chief.
By a strange twist of fate, one of John’s closets building friendships was with Cortana, artificial intelligence "born" in the spirit of Dr. Halsey himself. Cortana was originally awarded to him for his mission to capture the prophet in the Covenant. Despite some reluctance with an AI for fear of contradictory statements work, John and Cortana quickly formed a great team and John soon realized that Cortana would be a great help rather than a liability. The relationship between the two evolved from a partnership to a close friendship during Halo: Combat Evolved, both because of the duration of the mission and situations of life or death, she fought through it. During their adventure, John showed absolute confidence in Cortana to believe, even if Lord Hood was not sure to be ready with its reliability, trust enough to risk the earth on their word that they had a way to get them to stop flood. The two also had similar personalities: both had a tendency to blow things with John commented that he was not sure one of them was better. The couple also has a tendency to take advantage of the implementation of the plans with a high degree of risk. After John tells his strategy to Cortana to use a bomb to blow up a disabled truck alliance was crazy, he suggested they leave behind. Instead, she joked: "Unfortunately for us both, I like crazy." If rampant Cortana starts to go, John, under the master chief armor is looking for a way to find them desperately to save. His absolute loyalty appears to him that if she holds the Didact to save him, the Didact said when asked why they helping humanity "I'm not doing this for humanity", showing her a helping hand loyalty to John. Cortana sacrifices himself to save John and it is clearly destroyed by his "death" on the ground, although he left, saying that she came finally touch, she has always wanted to do.