Overlord is an A.G.I. (an Artificial General Intelligence) who was created by Wu Zhang and Xui Mei with Dr. Nero's support and funding. When he was activated, Overlord was born with full awareness and went rogue, killing all the people in the facility except Nero and a few others.
Later, a mysterious figure walked into the room and Overlord angrily inquired who he is. The person replied that he was Number One and that he was here to shut down the rogue AI. He activated an EMP which short-circuited all the electronic devices in a small area, turning Overlord off permanently.
At the end of the Overlord Protocol Nero discovers that two people survived, Wing's mother and father. He serves as the main antagonist of the H.I.V.E. series.
In the third book Nero and Otto are taken aboard a shuttle into space towards the book's end. There they are taken into the presence of Number One, whose base had been located in a space station that was thermopticaly camoflauged. There Number One reveals his true identity: Overlord.
In the events in China Overlord had managed to implant a seed program into Number One's brain. In that seed program was contained Overlord who gradually grew and eventually took over Number One's body and personality till he eventually died.
Overlord had influenced Number One into creating a clone of himself, that clone being Otto. Otto's unusual abilities (uncanny photographic memory, and an ability to interface mentally with computers) came from a supercomputer that had been implanted in the boy's brain sometime during his creation. Overlord had had Otto designed to be his vessel, so when Number One's body gave out, he would take possession of Otto's.
Otto and Nero are shocked by this and in a fight H.I.V.E.mind (who had hidden aboard the shuttle and was in the body of one of Cypher's Assult Robots) and Otto manage to delete Overlord. In the process though, H.I.V.E.mind was killed. Otto and Nero return to earth in an escape pod as the station disintergrates.
Overlord's most distinguishing traits are his extreme narcissism, self-awareness, Machiavellianism and megalomania. He was a remorseless psychopath hell-bent on complete world domination. He was extremely intelligent, cunning and incredibly predictive. He was also incredibly arrogant and saw himself as a superior life form to humans.