Ketsu Onyo is a bounty hunter contracting for the Black Sun syndicate, introduced in the second season of Star Wars Rebels and voiced by Gina Torres. StarWars.com frames her entirely in relation to Sabine Wren: "an old friend of the rebel Sabine, with both attending and escaping the Imperial Academy together. But they eventually took different paths."
The road not taken
That is her structural purpose, and it is a good one. Two young Mandalorians walk out of the same Imperial Academy with the same skills and the same grievance; one joins a rebel cell and the other joins a crime syndicate, and the show is careful not to pretend the second choice was irrational. Black Sun paid.
Blood Sisters
Their reunion is a job collision — both are after the same droid courier — and it resolves not through a change of heart but through an Imperial attack that neither can survive alone. Ketsu helps, takes the win, and then explicitly declines to join the rebellion. She flies the Shadow Caster and carries a staff, and she leaves as she arrived: employed.
Later
She turns up again in the fourth season, running supplies for the Rebellion on a contractual basis rather than a moral one — which is a more honest account of how insurgencies get their logistics than most of the franchise manages.