He, or she, is a Catalan cross between Banksy and an anti-tourism league.
At night the mystery graffiti protester roams the streets of Barcelona's old quarter leaving behind markings on the streets that divide them into separate lanes for the tourists and the "normal" Barcelonans.
Every day the city hall sends out workmen to paint over the markings, afraid that someone is trying to push away the tourists who bring in welcome wealth but increasingly make native Barcelonans feel jostled off their own streets.
The paintings have so far appeared in three streets in the Gothic old quarter.
One was found on the corner of Ferran and Avinyo streets - home to the prostitutes who inspired Pablo Picasso to paint his famous collection of naked women, Les Demoiselles d'Avignon.
A Picasso museum, housed in a medieval palace not far from the art school where the great painter trained, is among the draws that help make this one of the busiest tourism zones in Europe.
Two more street paintings were found in Carrer del Call and the Baixada de Sant Miquel - both in the tightly packed Gothic district, where tourists and local residents vie for space on the narrow, twisting streets.
Barcelona's security councillor, Assumpta Escarp, who also represents the old quarter, has called for the mysterious protester to come forward and explain what it is all about.
"They should be brave and explain exactly why they want segregation," she said.
The anonymous street paintings have set the city discussing not just the graffiti tactics of the protester, but also the limits of tourism in a city that sometimes feels set to burst.
Budget airlines, stag parties, a thriving nightlife, 5km of beach and the city's famously liberal atmosphere have all combined to turn its famous and colourful Las Ramblas boulevard into a heaving mass of slow-moving, camera-toting or beer-swilling foreigners.
A majority of the quarter of a million people who walk past the flower stalls and street painters on the bustling boulevard every day are no longer from Barcelona itself. With a 15 per cent surge in visitors expected this year, the problem of overcrowding can only grow.
"We cannot put up with much more of this in the neighbourhood," said Maria Mas, president of the old quarter residents' association, who complained the city hall had failed to put into place a plan to balance out the commercial use of the area, so that residents were not pushed out. "This at least makes us think a bit."
Attempts to attract more upmarket tourism and to repel the pink-skinned, shirtless northern and central Europeans in bathing trunks who so offend the local population appear to have had little success so far.
Police have moved to clear up the nightly spectacle of dozens of young Nigerian prostitutes on the Ramblas who had been openly harassing men and selling sex services in the alleyways beside the famous Boqueria market.
Now Barcelona's city hall is making yet another attempt at taming the wilder habits of the budget travellers who inundate the city every summer.
A poster campaign running on the city's buses and metro system urges them, quite literally, to keep their shirts on.
"A nudist beach is one thing; it is quite another to walk around naked wherever you want," complained Xavier Trias, leader of the opposition Convergence and Union party. "It is a question of common sense."
The campaign follows increasing local disgust at the behaviour of tourists who treat the city in the same way as they do the more popular beach-and-booze tourism resorts in Ibiza or the Costa Brava.
The announcement several years ago that foreign visitors would be given on-the-spot fines of up to £1000 (NZ$2084) for bad behaviour has failed to tame the incomers.
Most locals are not, however, in favour of segregating the tourists by making them stick to their own pedestrian lanes.
"Now that there is an economic crisis, we need tourism more than ever," old quarter shopkeeper Angeles Aldia complained to the local Vanguardia newspaper.
"The best solution is to widen the pavements wherever you can," said Ole Thorson of the Catalunya Walks group.
"It is true that sometimes the different rhythms at which people walk can bother people, but there is no need to bring in regulations."
Online debate about the paintings concentrates as much on the fact that the paintings are in Castilian Spanish, rather than Catalan, as it does on the tourism issue.
"Perhaps what people really want is three lanes, one for Catalan speakers, one for Castilian speakers and another for tourists," joked one wag on Vanguardia's website.
The Barcelona protesters appear to be emulating a group of New York performance artists, Improv Everywhere, who recently pretended to be city officials and painted lanes on Fifth Avenue, filming the reaction of locals.
New York's mayor, Michael Bloomberg, called the initiative "cute", but warned that many people in the city lived off tourism.
- OBSERVER
Barcelona: Prankster draws a line with tourists
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