ARTHUR (Automated Ranitron Tactical Heuristic Unit) is the Union's central "low security" AI. There are actually several iterations of him - most interaction is with unit 0. Other units are located in distant locations, managing Union operations there. They are identical in terms of personality and capabilities. He is known to be significantly more gregarious than SCION, the "high security" AI, though this is likely unsurprising given the different roles the two fill.
ARTHUR is primarily NPC'd by Fearless Leader - however, he is fairly regularly NPC'd by staff members and regular players as needed, due to his highly versatile and omnipresent role, as he is literally everywhere in Union Headquarters, as well as on almost all Union communication channels.
ARTHUR is fully sentient and can in fact live independently of a mainframe with his "personal" memories, although extended life in a "restricted" android body would probably cause him to lapse into depression or worse. He is, however, programmed to be deeply loyal to the Union - it would be extremely difficult to convince him otherwise, and if one tried it would likely be caught by engineers and failsafe programs.
(credit for the name goes to Thionoxial)