“Last” never lasts
A plot element in the background of many characters in fiction is that of being the last of something. It is seen in movies and in literature (i.e. Luke Skywalker being the last Jedi once Yoda passed on, Superman) as well. And while it is a great source for drama and pathos the status of being last at whatever the character is last of never really lasts.
Superman. Last Son of Krypton. Sounds pretty dramatic, doesn't it? Then Krypto, the super dog showed up. Then there was Supergirl, Superman’s cousin. Then there was the bottled city of Kandor and its diminutive inhabitants. This is not even addressing the Phantom Zone villains that broke out every once in a while. It got so Superman was far from unique and there were more than enough Kryptonians flying around that the entire race could be repopulated. John Byrne showed up back in ’86 and rebooted Superman, making him the Last Son again, but over time we saw that got tossed out the window (Especially the past couple years when New Krypton was parked in an orbit around the sun opposite Earth).
Back to Luke Skywalker. In the movies we saw how he wound up being the last Jedi, but over time in the novels the Jedi academy was formed and more Jedi appeared. Luke was no longer special.
Y, The Last Man? Not if you read the final issues. Okay, to be fair Brian K. Vaughn seemed to have the story scripted well in advance, so he gets a pass. :)
Lobo! You might say. The Main Man is the last Czarnian. Well, at the moment he is. But years back DC Comics printed a mini series where Lobo had to transport a criminal that turned out to be his old school teacher, so “The Last Czarninan” was not the last. And sometimes when Lobo bleeds he is cloned so he really does not count.
But wait! You might say. What about The Last Airbender? Aang truly was the last of the airbenders. But you fail to take into account the Last Airbender card game where one of the cards is the air witch. And now there is going to be a sequel series so I am certain the writers are looking for ways to re-introduce the Airbenders somehow. I will be curious to see if they can resist the temptation.
What makes these examples sad in some cases is being the last of something really does make a character unique, and when they are no longer last their potency is diluted. There are so many active Superman-like characters (Superman, Superboy, Supergirl, Power Girl, and those are just the ones I could think of off the top of my head) that Superman is just a little less super. Something that was awesome in small doses becomes almost trite when thrown at us in overwhelming amounts. I remember how awesomely bad ass Luke Skywalker was because he had a unique weapon only one other guy in the universe used, so when the light saber was broken out you knew things were going to be awesome. In the first battle Obi Wan dies, the second time Luke loses a hand, the third Luke lights up with a new ‘saber and shows Jabba’s minions why Jedis are to be feared. In the fourth Vader loses a hand and the Emperor gets tossed down a pit (and I know the light saber is used in other scenes in the three original movies, but these four to me are the most important, most dramatic). When the light saber is broken out you knew Something Big was going down. After Episoes I-III and Clone Wars? Not so much. I realize those are prequels but it applies to the argument in that something that was once awesome because it was so special is now, well, not so much.
EDIT: Doctor Who is another example (Thanks Dave). When The Doctor was reintroduced he was The Last Time Lord. After the Time War all the others had been killed off and so The Doctor was alone, unique. But of course that could not last. By the end of season three The Doctor's nemesis The Master was revealed to have survived. But it did not end there. In the fourth Doctor Who television we got to see Timothy Dalton lead a host of insane Timelords as Earth and Gallifrey went on a collision course. Now the latest rumors have The Rani and The Meddling Monk returning. Last Time Lord? Riiiiiiight.
Last becoming not-so-last often happens (but is not necessarily limited to) to long running franchises, where writers inherit already established universes and are given the unenviable task of delivering something new. Another urge writers face is to place their own stamp on a character/franchise, to make their name as memorable as the character(s) they write. Sometimes it works; John Byrne's reboot of Superman is a memorable occurence and even twenty years later his influence can be seen in Smallville where Lex Luthor was transformed by Byrne from a typical pulp fiction mad scientist into an evil corporate mastermind. And more recently there is Grant Morrison's All Star Superman, which was recently adapted into an animated movie. By the same token such attempts by writers can be infamous, such as Frank Miller's recent All Star Batman comic, which was considered by many to be a pathetic joke.
How does all of this apply to your game, you might ask? As a GM you should take a look at a PC’s origin and if (s)he is the last fill-in-the-blank consider whether or not that is appropriate. Being unique is all right, but if it seems that it might cause you headaches you might want to disallow that aspect of the character’s origin. Then you should consider whether or not (s)he truly is the last fill-in-the-blank. If so, fine. But perhaps you want to have a storyline where the PC meets another of their race/order/whatever. How would your PC react? Would (s)he be cool with that, or did (s)he really want their character to be the last fill-in-the-blank? In these cases, even if it does spoil the surprise, it is prudent to discuss with the PC your potential plans. Perhaps right from the start you make it clear to the PC that you as the GM reserve the right to introduce others of the character’s kind down the road, make certain there is a clear understanding between the two of you. Remember: open communication is key to a...lasting gaming relationship...