Jack is an AI robot ally of the COG soldiers. He does not speak and only shows up in a limited role, but he is very helpful. Jacks name comes from the term "Jack of all trades". Jack is used to "rip" open locked doors and is also capable of downloading and storing information in his CPU. He is able to follow alongside the COG by using a cloaking device to hide himself from view. Although not shown in-game he does come equip with tools to allow for field surgery. Another tool that Jack contains is a can-opener, presumably to make life for gears more bearable in the harsh battlefields. He has his own special compartment in Delta's Junker APC that they get from Franklin. He remains docked at all times until he is needed. He can break through doors with his torch and also serves as the communication and data connection between the squad and Command Control.
Jack is invulnerable in the game and the player is unable to play or interact with him at any time. Usually Jack is used as a basis for some game play elements. Whenever Jack is needed to open a door or download information there is usually always an attack from the Locust, Jack will never fully open the door until all of the Locust are killed.