Here is an interesting comparison of Cortana over the course of the 4 halo games:
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Buck from ODST
Level: New Alexandreia
In the mission New Alexandria you will be told to escort a falcon to a tower where a group of ODST's are in a "firefight". A "DEFEND" waypoint will over over the falcon. In the falcon is Gunnery Sergeant Buck from Halo 3 ODST. When you escort the falcon to its destination, Buck will exit the falcon walk into a room say "I owe you one spartan" and vanish.