In fiction a large amount of character who are supposed to be unusually long lived and immortal characters who now inhibit American have a tendency to come from Great Britain. Those who aren't tend to be from Eire.
At first glance this would seem make sense since America is generally regarded as a land built by migrants. However in some cases the characters are still 'young' enough to have been born and raised in America... which would have spared many weak accents.
This may be explained by the large number of inhibitants of America who acknowledge themselves as being descended from one of the two islands. It could easily be some bizzare form of wish fullfillment for them that someone who knew their great grandfather in their early twenties may still be alive and kicking.
Ofcourse in more recent works it could be related to the fact that the celtic and saxon races are the most reasonable ones to explain a lot of the prominent features in the appearance of American actors. Not to mention the fact that the countries in question already speak English makes for both an easy handwave solution and makes it all the more impressive when the immortal in question knows a multitude of langauges due to their longevity.
Of course it doesn't really count as this if the setting for the story is on one of the islands. This is more about the general prevalance of immortal characters from them when characters from virtually any country would make about as much sense.
Live Action Television
- From the Buffyverse we have atleast Angel, and Spike.
- Averted with Darla, who despite having being sired from a British woman is 'born as a vampire' (so to speak) in America.
- Sanctuary gives us Helen, John, James, and Adam.
- The various MacLeods in the Highlander franchise are Scottish.
- Macbeth and Demona from Gargoyles are Scottish. Though Demona isn't human.