James Ellison (Terminator)
In the context of the series, James Ellison was first an antagonist of Sarah Connor and then an ally. Skynet sent a Terminator to kill and replace him, probably both to get to Sarah Connor and Catherine Weaver, a T-1001 (slightly more advanced T-1000 with its chip set to read/write) that had gone rogue. It failed.
Great series, wish it would come back for another season.