Root

Root is a high IQ computer hacker. In season 1&2, she is the bad guy who tried to catch Finch to know more about the Machine. Her real name is Samantha Groves, when she was a child; her best friend was kidnapped and killed. She saw the man who did those things but no one belief what she told. After she grew up, her punished that man by hacking in the account of a boss who was in the dark road. She made that boss belief that it is the killer who stole the money belong to him so that boss send people to kill him. Root always uses this way to destroy the person she doesn’t like.

In season 3, Root started being in the group of the Machine, due to the Machine gave her power to directly talk to it. (Reese has this power too, but Finch only can get the massage indirectly because when the Machine was reset he let Reese to get that power.) So, now they are all in the same group to fight with the other AI, and try to make the social go back to past age.