Londo Mollari is the ambassador of the Centauri Republic. Londo was assigned to Babylon 5 because nobody else wanted the job. His time on Babylon 5 was first spent having fun gambling, drinking and being a general nuisance.

He is also controlled by the Shadows (I think) which leads to the downfall of the Centauri Republic. Which you could see in the film Babylon 5 : In the Beginning.

And somehow he becomes Emperor of the planet.