Rosa is a freelance agent.
Rosa, as noted in the fact sheet above, is not his real name, given or surname. That certainly exists but what it is we do not know and never shall for Rosa, as is the codename for this independent operative, much prefers his anonymity.
By that 'nickname' and by his speech when not in disguise, Rosa could easily be of Spanish descent. However, based on the brief history we get of him in the second adventure, it could be either from his upbringing or just a persona he enjoys.
Rosa is 32 years old in the second adventure and 35 in the first, having been born in 1835 to parents fleeing the "shame" of an unspecified scandal which destroyed the reputation of his father, an ambassador from "one of the most powerful nations on earth". When the infant was born, his parents lived as expatriates traveling the world which allowed the young boy to learn "the ways and languages of many of its nations".
It was in a Balkan country they were visiting when the boy was becoming a teenager that rebels attacked the family simply because they were apparently well off. His parents were killed and he barely escaped. What followed would be a few years living on the streets while learning how to survive.
We learn through a brief flashback how the newly adult man would impressed to fight for the British in the Crimea and how he deserted because he neither served the Crown nor hated the Russian. He joined a traveling circus and was told many extremely useful skills that would serve him well in his later profession.
Rosa is a skilled swordsman and not at all hesitant to pull his weapon to engage a foe in mortal combat, confident that his superb skill will prove victorious. He is also an adept at disguise and carries in his bag of tricks the "grease-paints and dyes" often needed to hide his normally dark features as he takes the role of someone else.