Taken from the Star Wars Wiki

Jaster Mereel was a male Human born on the Outer Rim world of Concord Dawn. Mereel served as a Journeyman Protector, one of the planet's respected lawkeepers, until the day when his strong morals led him to kill a corrupt superior officer. Charged with murder, Mereel was exiled from Concord Dawn, going on to join the Mandalorians. An accomplished soldier, Mereel's prowess in battle earned him the respect of his supercommando peers, and he would eventually go on to become Mand'alor, the leader of all the Mandalorian clans, and the Al'Ori'Ramikade—"Commander of Supercommandos". As Mand'alor, Mereel issued a new guideline for Mandalorian behavior known as the Supercommando Codex in which he asserted that the Mandalorians who wished to fight would simply be highly-paid soldiers, and should conduct themselves as honorable mercenaries.

Mereel would face opposition to his reforms both from the peaceful New Mandalorian faction, and the more dangerous Death Watch, a violent splinter group formed by the followers of Mereel's rival, Tor Vizsla. A civil war broke out between the Death Watch and the supercommandos loyal to Mereel, the early years of which would see Mereel's return to his home planet, where he saved the life of a local farmer's son, Jango Fett, when the Death Watch attacked the boy's home. Taking Fett in following the death of his family, Mereel raised the boy as a his son until the fateful Battle of Korda Six. Ambushed by the Death Watch and betrayed by his second-in-command, Montross, Mereel was killed by Vizsla. His body was carried back to the Mandalorians' landing zone by Fett, where Montross was exposed as a traitor and Fett succeeded his adoptive father as the new Mand'alor, in accordance with what Mereel would have wanted.

Jaster Mereel was a male Human born on the Outer Rim world of Concord Dawn. Mereel served as a Journeyman Protector, one of the planet's respected lawkeepers, until the day when his strong morals led him to kill a corrupt superior officer. Charged with murder, Mereel was exiled from Concord Dawn, going on to join the Mandalorians. An accomplished soldier, Mereel's prowess in battle earned him the respect of his supercommando peers, and he would eventually go on to become Mand'alor, the leader of all the Mandalorian clans, and the Al'Ori'Ramikade—"Commander of Supercommandos". As Mand'alor, Mereel issued a new guideline for Mandalorian behavior known as the Supercommando Codex in which he asserted that the Mandalorians who wished to fight would simply be highly-paid soldiers, and should conduct themselves as honorable mercenaries.

Mereel would face opposition to his reforms both from the peaceful New Mandalorian faction, and the more dangerous Death Watch, a violent splinter group formed by the followers of Mereel's rival, Tor Vizsla. A civil war broke out between the Death Watch and the supercommandos loyal to Mereel, the early years of which would see Mereel's return to his home planet, where he saved the life of a local farmer's son, Jango Fett, when the Death Watch attacked the boy's home. Taking Fett in following the death of his family, Mereel raised the boy as a his son until the fateful Battle of Korda Six. Ambushed by the Death Watch and betrayed by his second-in-command, Montross, Mereel was killed by Vizsla. His body was carried back to the Mandalorians' landing zone by Fett, where Montross was exposed as a traitor and Fett succeeded his adoptive father as the new Mand'alor, in accordance with what Mereel would have wanted.