LEON ILLICH PROTZNEDVIC

There are a number of Leons out there. In fact there is an entire planet full. Here are some we have observed:

 

Leon the Postman
This is the Leon that Jim met on a spaceship in Episode 9. He claims to have no knowledge of Glecon 7. He describes himself as a Postman of the Royal Mail (as opposed to a Postman of the Apocalypse). He suggests that his intentions were to observe the Kleeks of Yavill, but he also suggests that he would rather do something else. So presumably this is a job rather than a hobby. How it fits with postal work is unclear.
It may be that Leon's lack of awareness of Glecon 7 etc. is due to some kind of memory loss. It may, however, be that Leon's lack of awareness of Glecon 7 etc. is due to such things coming later from the perspective of this said Leon. It could be that this Leon precedes (referentially rather than temporally) the other Leons we have come across. Opinion is divided on the matter.
In our (rather privileged) experience of events, we see this Leon going back in time with Jim. But later he is erased from this time-line as a consequence of a paradox. So presumably he went on to never meet Jim, which may well mean he ended up observing the Kleeks of Yavill: little green alien students that get into all sorts of hilarious scrapes, apparently. This seems reminiscent of the task performed by a different Leon who we will come to examine shortly.

Darth Leon
The original Leon, who may or may not be the same Leon as the one outlined above. Darth Leon was one of the Four Postmen of the Apocalypse, until the collective were overthrown by Yoga and the Zen Flamenco Dancers. He then set into business as an assassin. The only job we ever saw him working on was an attempt on the life of his old colleague Darth Unpleasant, during which he masqueraded as a character called Rick Olé. It was during this ill-fated mission that Leon found himself caught in a malfunctioning cloning unit which produced thousands of carbon-copy Leon clones. The fate of the original Leon is very much lost in the confusion of this multitude of copies who went on to populate a small planet called Glecon (or Gleceon) 7.

Leon the Student
Leon claims to be from Yugoslavia. He claims to be a music student. Indeed, he is to be found in the halls of residence of York University where he is occasionally seen with a violin case and some music manuscript. However, his university colleagues believe he might be an assassin; a notion born out slightly by Leon's 15-1 fact-file in Episode 1. Why then is Leon at York Uni? It is later revealed that his role is as an observer (cf. Leon the Postman). Even later it is suggested that he was observing "the humans" to ascertain whether they were involved in "Intergalactic freud-hopping". What freud-hopping involves is unclear, and what Leon would've done if any evidence of freud-hopping had been unearthed is equally unclear, if not more so. Leon is certainly a trained assassin (perhaps Darth Leon's training was reproduced in the clones), but has he ever killed anyone? This incarnation claimed to have killed Barnie the cleaning-lady, but we subsequently saw that Ant killed her. Was this a rewriting of history? Did Leon kill her originally, only for Ant to go back in time and kill her before Leon got the chance (if so why?), or was Leon just lying/joking about killing Barnie? This is unclear.
This incarnation of Leon got blown up only to be reanimated by MI7 (what was their motivation for this charitable act?). He then got sucked into a parallel reality along with Alex and Stew. It was in this reality that he got killed by disco-dancing aliens.

Leon the Replacement
Clearly, at the time, there was more observing to be done, as a new Leon was sent down, arriving at Stew's funeral. This Leon seemed well versed on the previous Leon's existence and was able to take his place without difficulty. Either the Leons posses a collective memory/experience, or this Leon had been observing his predecessor one way or another. The latter seems more likely.
This Leon accompanied the rest of the team into TV Heaven and then later into TV static where they were entranced by the evil Fred Harris's Chocabloc. During a moment of lucidity, Leon is seen to report back to his superior on the freud-hopping, confirming that there isn't any going on. However, he suggests that Babysham is perhaps smuggling lead-crystal decanters, and is instructed to locate Babysham and put a stop to any such operations. Babysham soon turns up, and our heroes are torn between two separate realities. In one of the two realities, Babysham gets blown up on a beach, leaving Leon free to indulge in some high adventure on the high seas with his colleagues. He eventually kills himself as a result of Caroline's incessant quoting of Eddie Izzard. In the other (more real) reality, Leon et al find themselves back in York only to discover that Siddor has taken on their favourite pub. Siddor ends up killing everyone bar Leon, and in return, Leon shoots Siddor dead before beaming back home to Glecon 7. Presumably, the Babyham issue was dealt with one way or another.
Owing to the nature of Leons, we can not be 100% sure, but it was probably this Leon that was sent to Hell to tell his friends about the Geese problem and to help sort everything out. He was later shot dead by the psychotic Alan on board the Barbican.
However, as a consequence of Time Lord interference, this reality is later revised such that Leon doesn't get shot. Instead he gets splattered in a Tardis accident.
This dead Leon winds up in Hell. He later winds up running a book shop in Heaven with Stew, before getting dragged back to the Barbican by a book. He ends up on Planet F34, his future looking uncertain after he knocks Sarah Michelle Gellar out with a shovel.
Evidently things are sorted out, and Leon returns to the bookshop with Stew. Both of them agree to help Babysham with some theological issues, during which Leon seemingly assasinates God, Bhuvana and Satan, using some special god-killing ammo. He is pursued from the scene by JimSD but gives him the slip before dramatically ascending into a flying saucer.

Leon the Superior
May be two different Leons, but this is the Leon seen giving instructions to Leon the Replacement on Glecon 7.

Leon the Observer
Another incarnation of Leon crops up in the Caribbean episode. Here he is employed by Tomy to monitor the quality of the storyline. Again, he seems well versed in the experiences of the other Leons. It makes you wonder if there's been a number of Leons following everyone around all along. Well maybe not.