The Green are a species of aliens, also referred to as "The Occupiers" or just "Occupier." It seems they take a host inside of a living being's body, or that of a corpse. After the host is infected, The Green assumes total control of the body, as well as the mind. They seem to have some form of visual hypnosis that they use to lure victims into a trance. This hypnosis can be fought off with enough willpower. It is revealed in the sneak peak at the end of Lifeline (1) that when an occupier takes over a body, it can utilize the memories of the host and use it to their advantage.
In order for them to "possess" or enter a person, they must enter directly into the mouth of their chosen host. The Green can improve the strength, as well as the speed, of the host once they become occupied. As revealed in Lifeline: Flatline, the Green can be suppressed by a chip manufactured by the Permanent Working Group.
[SPOILER] The Green-Eyed Woman from Lifeline: Crisis Line seems to have the ability to freeze time and project the memories of humans.
Throughout the series, the Green "occupy" a variety of living things, such as rats in Lifeline; humans in every game except Lifeline: Whiteout; dogs and apes in Lifeline: Flatline; and squirrels and flies in Lifeline: Crisis Line.
The Green do not appear in Lifeline: Whiteout.