Jodo Kast is a bounty hunter in Mandalorian armour who impersonates Boba Fett, introduced in the Dark Horse one-shot Boba Fett: Twin Engines of Destruction (1997).
Trading on somebody else's name
The premise is a good one and unusually specific to this setting: Fett's reputation is worth more than his skills, the armour is generic enough that anyone in a similar suit reads as him, and a hunter who lets clients believe he is Fett can charge Fett's rates. Kast does exactly that while Fett is recovering from the sarlacc and is in no position to correct the record.
Why Fett has to kill him
The story is set during Fett's recovery, and Fett hunts him for reasons that are commercial rather than wounded pride. A counterfeit who takes jobs badly damages the brand; a counterfeit who takes jobs well is worse, because he proves the brand is transferable. Kast's death is Fett protecting an asset.
Continuity
Kast is Legends. He recurred across Dark Horse comics and West End Games material in the 1990s and has not been reintroduced to current canon — though the underlying idea, that Mandalorian armour confers an identity that anyone can put on, is precisely the question The Mandalorian and The Book of Boba Fett spend their runtime arguing about.