Bo-Katan Kryze is a Mandalorian warrior of Clan Kryze, introduced in the fourth season of Star Wars: The Clone Wars in the 2012 episode "A Friend in Need" and voiced — and later played in live action — by Katee Sackhoff. StarWars.com calls her "a legendary Mandalorian" who "has been fighting for the future of Mandalore since the Clone Wars, wearing armor that has been in her family for generations".
She starts as the villain
Her first appearances are as a lieutenant of Death Watch, the terrorist group led by Pre Vizsla that is trying to overthrow her own sister, the pacifist Duchess Satine Kryze. This is not softened. She is complicit in the campaign that destroys Satine's government, and she is on the wrong side of it for years of in-universe time before she changes her mind.
The Nite Owls and the split with Maul
When Maul arrives, absorbs Death Watch into his Shadow Collective and kills Vizsla in single combat, Bo-Katan refuses to follow a Sith and leads the breakaway Nite Owls against him. It is the first genuinely principled thing the character does, and it costs her: she spends the rest of the war fighting a civil war she cannot win, and ends the Republic era allied with Ahsoka Tano in the Siege of Mandalore.
The Darksaber, refused and then lost
In Star Wars Rebels, Sabine Wren wins the Darksaber and offers it to her. Bo-Katan initially refuses, believing herself unworthy after everything Death Watch did, and only accepts when the assembled clans acclaim her. She then loses it — and with it the claim to rule — when the Empire purges Mandalore and Moff Gideon takes the blade.
The problem the tradition creates
That loss is the engine of her arc in The Mandalorian. Mandalorian custom holds that the Darksaber must be won in combat; a gift is worthless. So the woman with the best claim to rule cannot legitimately be given the throne by Din Djarin, who won the blade by accident while fighting an Imperial. The show takes three seasons to work out that the only honest solution is a fight staged in front of witnesses.
Two kinds of Mandalorian
Her other function is doctrinal. Bo-Katan removes her helmet freely and regards the Children of the Watch as a cult of religious zealots; they regard her people as apostates who lost their world. The series treats this as a real argument with two defensible sides, and her eventual willingness to bathe in the Living Waters — and to fight alongside the covert — is the reconciliation the whole Mandalorian storyline has been building toward.
Why she is unusual
Very few Star Wars characters are allowed to be wrong for this long. Bo-Katan spends roughly thirty years of in-universe history changing sides, losing wars, taking responsibility for atrocities she helped cause, and being handed authority she does not think she has earned. It is the closest the saga comes to a political career.