"The Scarlet Pimpernel, Mademoiselle," he said at last, "is the name of a humble English wayside flower; but it is also the name chosen to hide the identity of the best and bravest man in all the world, so that he may better succeed in accomplishing the noble task he has set himself to do."

A novel about wealthy sniveller Sir Percy Blakeney is doomed and a novel about a dashing counter-revolutionary might soon be forgotten, but a novel about a freedom fighter who disguises his true mettle underneath a foppish exterior is pure gold.

They sought him here, they sought him there, those Frenchmen sought him everywhere, but they never quite figured out that the damned annoying Scarlet Pimpernel was neither in Heaven nor in Hell but right under their noses. Baroness Orczy wrote other books, but none as popular as The Scarlet Pimpernel; perhaps it was the scintillating prose or perhaps everyone really likes rooting for a swashbuckling Englishman who fights for the French aristocracy, but I think the answer is more simple:

"The Scarlet Pimpernel, Mademoiselle," he said at last, "is the name of a humble English wayside flower; but it is also the name chosen to hide the identity of the best and bravest man in all the world, so that he may better succeed in accomplishing the noble task he has set himself to do."

A novel about wealthy sniveller Sir Percy Blakeney is doomed and a novel about a dashing counter-revolutionary might soon be forgotten, but a novel about a freedom fighter who disguises his true mettle underneath a foppish exterior is pure gold. As far as I know, Orczy’s book was the first to adopt the fop-by-day, swashbuckler-by-night convention; she was quickly followed by Johnston McCulley, whose useless Don Diego fought injustice in Spanish California by donning the mask of Zorro. A few years later, Siegel and Shuster gave the world Superman — who had a day job as milquetoast reporter Clark Kent — and shortly after that Bob Kane and Bill Finger took one part Zorro and two parts The Bat to create the star of Detective Comics, a frivolous young millionaire named Bruce Wayne. After that, it was a truth universally accepted that a superhero in possession of a costume must be in want of a good secret identity (although decades later, Frank Miller’s The Dark Knight Returns suggested — in between bouts of Reagan-era urban dystopianism, meditations on the nature of the vigilante, and Bats beating the snot out of Superman — that "Bruce Wayne" is the construct that Batman dons to make his crimefighting possible, rather than the other way around). While there are exceptions — nobody is in doubt about who the Fantastic Four are, for instance (and with Ben Grimm’s rocky hide, how could he keep himself undercover?) — most superheros have a secret identity that comes in one of three flavor: unappreciated nebbish (Clark Kent, Peter Parker); dissipated zillionaire (Bruce Wayne, Lamont Cranston), or two-fisted patriot tough guy ("John Jones", the Martian Manhunter, is an alien, but he works days as a hardboiled PI; Tony Stark got his start as Iron Man designing weapons for the American military to use in Vietnam). Maintaining your secret identity is a tough business, and it can be hard to keep track of who knows what, but there must be advantages.

These people aren’t anonymous like the author of The Pearl, the finest piece of verse in Middle English, or the Master of Flémalle. There’s a lot of ambiguity in the past — forgotten emperors of Britain, for instance (link via Girlhacker) — that people need to discuss, so historians attach names of convenience. And many authors use pen names, of course. Some have gone to some lengths to maintain them, although their efforts have rarely involving shapeshifting Martian duplicates or superhypnosis. Romain Gary was a major French writer who, under his other name, É Ajar, was an even more important French writer. Gary felt that he had become pigeonholed and needed to escape his identity; he loudly announced that he had no connection to Ajar, and went so far as to hire an Ajar impersonator to lead literary investigators astray. The year after his death, the impersonator revealed the trick, and Romain Gary’s secret identity was a secret no more. Corpus analysis — simply looking at the language used by the two authors — might not have been useful in discovering the truth, and given literary trends in the Seventies, looking for a preponderance of "whores and nymphomaniacs" in Gary and Ajar’s writings might not have done the trick either.

Sometimes, an identity springs into existance for other reasons. Nicolas Bourbaki was a towering figure of twentieth century mathematics, notably largely for his contributions to set theory and attempts to use set theory as a basis for all mathematics. Nicolas Bourbaki was never a person at all; he was a collective identity seized by a group of young Parisian mathematicians (including Andre Weil and Szolem Mandlebrot, whose nephew‘s work you may have seen) who adopted the Bourbaki identity as a sort of Economist-style collective voice for their rigorous philosophy. Mathematicians were forced to leave the group at fifty, and new blood was ushered in; although the Bourbaki group still exists, Nicolas Bourbaki has not published anything in over twenty years thanks, in part, to a legal battle over publishing fees.

The Scarlet Pimpernel chose his name for his personal safety, to travel under the cover of his ineffectual identity. How many Jacobins would quail before a man named Percy? (Given that a pimpernel is a sort of flower, his other identity hardly induces terror.) Actors do this all the time. The Screen Actors Guild mandates that members have distinct names, so people end up being called things like "Jm. J. Bullock", and there’s a long history of actors changing their names to sound less ethnic, the sort of thing that leads "Joseph Gottlieb" to become Joey Bishop. Sometimes odder reasons crop up. Nicolas Cage (who selected his screen name as an homage to Luke Cage, also known by his nom de cape, Power Man) to disguise the fact that he’s related to half of Hollywood, and the celebrated Alan Smithee is a secret identity for directors to use when they feel their work has been compromised. But for identities that really scream "dark avenger of the night", people should look to musicians. Richard D. James spins records and twiddles knobs as the sinister Aphex Twin (aka AFX, Polygon Window, and Power Pill)! Mild mannered theorist Paul Miller slips off the garb of an academic and becomes DJ Spooky, that subliminal kid! In the Seventies, people like Iggy Pop and Cheetah Chrome flipped identities at the drop of a hat, without even the excuse of limited edition crossovers and Secret Origins reprints.

The best example of someone inhabiting an assumed identity, however, may not be an entertainer at all. Back in Siegel and Shuster’s day, Superman was "the man of tomorrow", since the imaginatively named Steel Sterling was the "man of steel". That title already belonged to someone in the real world, however: the son of a poor but honest cobbler from Georgia. When he began his dangerous activities as a revolutionary, for his own safety he adopted the pseudonym "Korba," after an outlaw freedom fighter from a piece of popular, nationalist literature. But he needed his own identity, and a nickname for him eventually stuck. Very few people are gratuitously evil in their own minds, even when setting out to intimidate a superstitious and cowardly lot, but Joseph Vissarionovich Djugashvili, the "man of steel" or "Stalin", at least picked a name that suggested supervillainous qualities.