"Yes, captain. Or he is. It depends on your point of view, doesn't it?"
Lazarus was an antimatter version of Lazarus from an antimatter universe. Lazarus's civilization discovered a way to slip through the alternative warp that connected the two universes. When he made this trip, his existence was revealed to Lazarus, who became insane and obsessed with his destruction.
The Lazarus from the normal universe created an elaborate paranoid delusion in which his counterpart was a "hideous monster", a "murdering creature", and a "terrible thing". Lazarus claimed that his counterpart could compute mathematical formulas to destroy civilizations and entire races and that Lazarus had only escaped the destruction of his own society because he was inspecting magnetic communication satellites when "the Thing" had attacked. Lazarus further described his counterpart as a "Beast" who was "all white and black" with "a terrible emptiness".
Lazarus pursued his counterpart across time and space, eventually arriving in 2267, where their conflict drew the attention of Starfleet and others – because it threatened the existence of both universes. After learning the full story, Captain Kirk assisted by forcing Lazarus into the corridor and destroying Lazarus' spaceship. This action sealed the corridor and saved the universes, but left the two men trapped together for the rest of time. This was a price the antimatter version of Lazarus was willing to pay to save two universes. (TOS: "The Alternative Factor")