Carasynthia Dune is a human former Rebel Alliance shock trooper from Alderaan who becomes a mercenary and later a New Republic marshal on Nevarro. She first appears in The Mandalorian episode "Chapter 4: Sanctuary" (2019) and is played by Gina Carano.
Alderaan
Her homeworld is the one the Empire destroyed in A New Hope, which makes her one of the few characters in the franchise defined by that event from the survivor's side. She fought through the war, took part in mopping up Imperial remnants afterwards, and then found that demobilisation left her with a skill set and no purpose — so she took security work in the Outer Rim, which is where Din Djarin finds her.
"Mandalorian isn't a race"
She gets the line that the series' whole treatment of Mandalorian identity rests on, and it corrects an error a lot of the audience arrives with. Mandalorian is a creed and a culture that adopts foundlings of any species — not a species. The encyclopedia follows that distinction throughout: Din Djarin, Bo-Katan Kryze and Sabine Wren are all recorded as human.
Production note
Carano was dismissed by Lucasfilm in February 2021 and the character has not appeared since; a planned Rangers of the New Republic series was cancelled. Her role as marshal of Nevarro is written around in The Book of Boba Fett, where she is said to have been recalled. This is recorded here as production history, not as in-universe fact.