Kir Kanos is a human Imperial Sovereign Protector and the sole survivor of Carnor Jax's massacre of the Royal Guard on Yinchorr, created for Dark Horse's Crimson Empire (1997). He is a Star Wars Legends character and the closest thing the Expanded Universe has to a ronin.
The oath
Kanos's entire arc is an argument with himself about a promise. He swore to protect Palpatine; Palpatine is dead, murdered by a fellow guardsman; and Kanos concludes that the oath now obliges him to kill everyone responsible. That includes Jax, the Imperial Ruling Council, and — for a considerable stretch of the story — Luke Skywalker.
Kenix Kil
He works under the bounty-hunter alias Kenix Kil while hunting his way up the list, and forms an increasingly complicated relationship with the New Republic officer Mirith Sinn. The sequel series, Crimson Empire II: Council of Blood (1998–99), is largely about that erosion: a man discovering that the loyalty he built himself around was to an institution that used him.
The end
When the Restored Empire's Ennix Devian plans to destroy Coruscant with zinethium, Kanos sets aside his oath against Skywalker to stop it, is framed for the attack, and defeats Devian anyway. Badly wounded, he is rescued by Baron D'Asta, recovers, and stages his own death to walk away from the Empire entirely — which is the only exit the character was ever going to accept.