Nexus originated as a drawing a sketchbook. He's a great warlock with immense magical power and station in life, and selfish person who knows what he wants and will use what means he can to get that - and that, usually, being power.
He's popped up a lot over the years as I often recycle him as a villain in comics. Originally, after the sketchbook, I used him as the main villain in a Dungeons and Dragons campaign, in which he was nothing more than a wily dark wizard bent on resurrecting a lich and taking over a kingdom. Or something like that... anyways, he became the main villain in Axis, as well, in some respect spanning all 3 timelines that the comic did -- even if he was a wraith in two of them. He also was intended to be the main antagonist to Sartris in that comic; head of the magic school that trained Sartris, manipulated him into leaving behind what he loved, allying with things he didn't believe in, and even plaguing Sartris, too, from beyond the grave.
One tidbit of Nexus that has always stuck, since his original arc in Axis, is his ghostly form, based off the cursed man from the original sketchbook drawing and D&D campaign. A separate entry for that coming later!
Character Profile: Nexus