Ancient scientist who lived during The First Siege of Atlantis. When Dr. Elizabeth Weir, Major John Sheppard and Dr. Radek Zelenka used the Ancient time device to travel ten thousand years into the past the Ancients at Atlantis successfully recovered their sunken Puddle Jumper. Sheppard and Zelenka had apparently died on impact.
When Janus heard Weir's story he realized his time device was responsible for saving the small group and sending them into the past, giving them another chance to save the future Atlantis expedition. Janus took Weir to the council of Atlantis, who were shocked and angered when they discovered (after forbidding him) that Janus went ahead with his plans and built the time device.
Janus and Weir petitioned to allow her to use the time device and travel back into the future just before the terrible accident, but were forbidden by the council. Janus remained astonished that the ultimate plan of the Lanteans had worked: Atlantis was to remain preserved for 10,000 years. Even their human descendants had discovered the city.
Instead of using the time device to get Weir home, Janus reconfigured the three Zero Point Modules that powered the city to rotate one by one. Approximately every three thousand years Weir would have to awake from her slumber in an Ancient stasis unit and rotate the power consumption herself. As a final fail safe -- in case she wasn't revived when the expedition team came through -- he programmed Atlantis to un-clamp from the ocean floor and rise to the surface.
Upon his departure to Earth Janus promised Elizabeth he would try to build another time device. It is believed that he did what he promised: SG-1 eventually discovered another Puddle Jumper with a time device on Arkhan's planet in the Milky Way Galaxy. A tablet explaining the planet's history was written in Ancient, possibly by Janus himself, who left the technology on the surface, likely as he went to live with those who would eventually become Arkhan's people.
Janus also left behind a secret laboratory hidden within Atlantis, which would be discovered by Rodney McKay and Daniel Jackson. The technology inside included the genocidal Attero device, leaving Janus with a morally complicated legacy.
It's Good To Be King - Janus's new time device is discovered in an abandoned Puddle Jumper on Arkhan's planet and is used to defend the world from the Goa'uld System Lord Ares.
Before I Sleep - Janus revives Dr. Weir after she has crashed into the ocean inside a Puddle Jumper, astonished that his technology does indeed work.
First Contact - Rodney McKay and Daniel Jackson discover a secret lab Janus kept in Atlantis, for secret and controversial experiments.