Colonel Ari Ben Zayn was a member of Earthforce Internal Affairs, at least until the attempt to usurp Jeffrey Sinclair's post as Commander of Babylon 5.
He and agent Harriman Gray were on Babylon 5 during 2258 to investigate the loyalty of the Babylon 5 staff. In fact, the investigation was arranged by Psi Cop Alfred Bester, who had a grudge against Sinclair after he killed one of Bester's men.
Ben Zayn originally applied for the post as Commanding Officer of Babylon 5 but was rejected because the Minbari wanted Sinclair instead. Since that day, Ben Zayn tried to get the command, in the strong belief he would deserve it.
His attempt to dispossess Sinclair failed as Agent Gray was convinced to scan Ben Zayn instead of Sinclair and realized the plot that Bester and Ben Zayn had set up.[1]