As a child, the only time he saw outside was when he crawled through a duct in his family's cramped living space. He and his wife Sonia got lucky in the eugenics lottery and had a child, Mark. He was teaching at MexTech in 2146 when the American economy collapsed and there was rioting. He was able to escape on the last ship out, but saw his wife and son in the crowd of people who hadn't made it outside. For the rest of his life, he regretted not going back for them.
In 2152, he began working on Project Eden in the Lucifer system. He frequently had arguments with Christine Lafayette about the division of resources between their departments. Secretly, he created a simularity of his son, Mark.
In 2157, shortly after the Seventh Doctor, Bernice Summerfield, and Ace arrived on the Project, Bannen discovered the Mushroom Farm inside of Belial. He began to tamper with the alien controls that were operated by emotion, not fully understanding their purpose. The moons Belial and Moloch started to move out of alignment, and he couldn't fix it because Ace had stolen his notes. Eventually, Moloch's feedback mechanism corrected his error, but not before many of the people on the Lift, a space elevator between the two moons, were killed or injured.
When a fleet of Interplanetary Mining Corporation vessels invaded the system led by the multidimensional being Legion, Bannen switched sides and started working for it. Legion entertained his ideas, but had no plans to actually carry them out and just wanted to give Bannen the illusion of power. When he realised this, he went once more to the Mushroom Farm, now understanding its true purpose: modifying morphic fields. He made himself handsome and the simularity of his son real. Waves of reality-altering energy emanated from the Farm, causing gruesome deaths for the crew who found their biologies incompatible with their environment. The Doctor, Ace, and Bernice tried to stop him, but Bannen was too emotionally unstable to operate the controls. The trio used the emotions in their memories to stop the waves of energy, and Alex and his son's simularity became part of the feedback mechanism in the form of the vegetation within Moloch. (PROSE: Lucifer Rising)