Rosa, the Spanish swashbuckler of The Mermaids Call is something of an enigma. No one knows where she comes from heck, not even her. She can have a fiery temper but also a very quick sense of her surroundings and situation.  That's why Captain Sedi uses her as her tactician, if they are in a pickle Rosa is the one who will get them out. 
 
She seems to have a raw infinity to wind and can use it to aid her in her attacks with her sword. It's said she can swing her sword so fast that it can slice you from a distance with only the wind from her strike. 
 
In the end, Rosa loves Captain Sedi after all she is the one who saved her from the streets in the Spanish iles of Serpiente.