He was also the co-creator of the post-apocalyptic science fiction strip Axa, as well as a substantial body of work in his native Spain.

[4][5] Romero began his association with writer Peter O'Donnell's Modesty Blaise strip in 1970 when he was called in to finish the storyline The War-Lords of Phoenix due to artist Jim Holdaway's unexpected death.

[1] Romero drew the Modesty Blaise strip until 1978, and while doing episodes of André Chéret's Rahan for the Franco-Belgian comics magazine Pif gadget from 1976, and beginning in 1978, Romero collaborated with Donne Avenell to create the science fiction series Axa for the English tabloid newspaper The Sun.

In 2002, Romero was commissioned to draw a graphic novel adaptation of the Modesty Blaise short story "The Dark Angels"; this work was initially published exclusively in Scandinavia but was later reprinted in a special issue of Comics Revue in the United States.

Romero's entire run on the Modesty Blaise strip is continually reprinted in an ongoing series of compilation volumes published by the UK company Titan Books since 2005, while Comics Revue in the USA has reprinted all of his post-1986 work on the strip.