A male Human, Jaster Mereel hailed from the Outer Rim planet of Concord Dawn. For a number of years, Mereel honorably served as a Journeyman Protector, one of the esteemed law enforcers of his world, until his strong sense of right and wrong compelled him to kill a dishonest officer who was his superior. Accused of murder, Mereel was banished from Concord Dawn, after which he became a member of the Mandalorians. A highly skilled warrior, Mereel's combat skills earned him the respect of his fellow supercommandos, and he eventually rose to the rank of Mand'alor, the supreme commander of all Mandalorian clans, and the Al'Ori'Ramikade—"Commander of Supercommandos." In his role as Mand'alor, Mereel established a new code of conduct for Mandalorians known as the Supercommando Codex, stipulating that Mandalorians who desired to fight would function as highly compensated soldiers and should behave as honorable mercenaries.
Mereel's reforms were met with resistance from both the pacifistic New Mandalorian faction and the more menacing Death Watch, a ruthless offshoot group founded by followers of Mereel's rival, Tor Vizsla. A civil conflict erupted between the Death Watch and the supercommandos who remained loyal to Mereel. During the initial stages of the war, Mereel returned to his birthplace, where he rescued the son of a local farmer—Jango Fett—when the Death Watch attacked the boy's residence. Following the death of Jango's family, Mereel took him in and raised him as his own son until the fateful Battle of Korda Six. Ambushed by the Death Watch and betrayed by his second-in-command, Montross, Mereel was slain by Vizsla. Fett carried his body back to the Mandalorians' landing zone, 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 desired.

Jaster Mereel, a male Human, was born on the agricultural planet of Concord Dawn, situated within the Outer Rim's Mandalore sector. As an adult, Mereel functioned as a Journeyman Protector—one of the respected lawkeepers of his home planet—until his strong moral principles led him to kill a corrupt superior officer. Subsequently, Mereel was accused of murder and exiled from Concord Dawn. Sometime later, Mereel joined the Mandalorians. He became a skilled soldier, and his martial capabilities earned him the admiration of his peers. By approximately 60 BBY, Mereel had achieved the rank of Mand'alor, the traditional leader of the Mandalorian clans. Observing the widespread savagery prevalent among certain warrior groups, Mereel sought to elevate the Mandalorians' standards of conduct. As Mand'alor, he established the Supercommando Codex, a new set of behavioral guidelines influenced by his own moral ideals. This Codex stipulated that Mandalorians who wished to engage in combat would be regarded merely as highly paid soldiers and should conduct themselves as honorable mercenaries. Around this period, Mereel engaged with the Regent of Mek va Uil, serving as both Mand'alor and Al'Ori'Ramikade—"Commander of Supercommandos." During this interaction, Mereel conveyed to the regent his perspectives on the essence of being a mercenary and the reasons they deserved respect.
Nevertheless, Mereel's concepts were not universally embraced within the Mandalorian community. By this time, a significant number of Mandalorians had aligned themselves with the peaceful New Mandalorians, a reformist political movement that emerged following a devastating conflict with the Galactic Republic centuries prior. The New Mandalorians had distanced themselves from the traditional warrior ethos, denouncing the ancient warrior codes, and believed that the best path to Mandalorian survival and prosperity lay in embracing peace, neutrality, and tolerance. Further dissent arose from a segment of amoral Mandalorians who resented Mereel's attempts to curb their lifestyles of irresponsibility. These discontented individuals rallied behind a Mandalorian warrior named Tor Vizsla, forming a splinter group known as the Death Watch, with Vizsla as their leader. They purported to restore the Mandalorians to their ancient role as conquerors and raiders, aiming to instigate another war of conquest. Despite the Death Watch's goals being diametrically opposed to those of the New Mandalorians, they could not support Mereel's faction of loyal supercommandos—now known as the "True Mandalorians"—because any act of Mandalorian violence fundamentally contradicted the New Mandalorians' principles. Mereel faced pressure from Mandalorian hard-liners, such as Montross, to eliminate the Death Watch, and civil war soon erupted between Mereel's True Mandalorians and the Death Watch. The New Mandalorians adhered to their ideals of pacifism and non-violence, rejecting both factions and remaining uninvolved in the conflict that became known as the Mandalorian Civil War.

In 58 BBY, the Mandalorian Civil War extended to Concord Dawn, prompting Mereel's return to his homeworld. Pursued by the Death Watch after a lost battle, Mereel and his men were compelled to seek assistance from the Journeyman Protector who had succeeded Mereel, a man named Fett. Fett provided them sanctuary on his family's farm and supplied them with food in the fields. However, the Death Watch captured Fett's young son, Jango, and attempted to coerce the Journeyman Protector into revealing the location of Mereel and his True Mandalorians, both through the boy's capture and a brutal beating. Fett refused to yield, and a chaotic firefight ensued when Fett's wife attempted to rescue her husband and son. Both Fett and his wife perished, but Mereel intervened—compromising his position—to save young Jango's life. As the Death Watch ignited the field into which Mereel and his troops had fled, Jango guided them to an irrigation pipe that facilitated their safe escape from the conflagration. Mereel regrouped his forces in a nearby town and awaited the arrival of the Death Watch, at which point he launched an ambush. While the Mandalorians engaged the Death Watch troops, Jango placed an explosive charge on the underside of the armored speeder upon which Vizsla rode into the town. The tank detonated, although Vizsla survived. As the battle concluded, Mereel discovered Jango standing over the corpse of the Death Watch soldier who had murdered his parents. Commending the boy's courage, Mereel decided to take Jango with him as the Mandalorians departed Concord Dawn, eventually regarding the boy as his son.

By 52 BBY, six years after the battle on Concord Dawn, Mereel believed that the Death Watch had been eradicated. With the Death Watch gone, Mereel led his True Mandalorian supercommandos as a mercenary force, and in that capacity, they traveled to the planet Korda Six. Hired by the Korda Defense Force, Mereel and his men were tasked with extracting a rookie security team pinned down under fire by local hostiles. Kordan intelligence suggested that resistance would be minimal, as the natives lacked a formal army and were poorly armed. Deeming the mission "easy credits," Mereel divided his men into three teams: Vertigo Company, an aerial unit utilizing jetpacks under the command of Montross, Mereel's second-in-command; "Jango's Grunts," which would provide cover fire and secure the extraction zone, under the command of a now-teenage Jango Fett; and Headhunter Company, Mereel's own unit, responsible for reaching the recovery site and extracting the security team. As the Mandalorians boarded their ships, Mereel spoke briefly with Jango, urging the boy to make him proud.

During the approach to the planet's surface, Mereel's ships sustained heavy damage from ion cannon fire, forcing most to make rough crash landings. The estimated "minimal resistance" proved to be grossly inaccurate, and the Mandalorians immediately came under intense fire. Mereel was helpless as numerous soldiers fell victim to enemy blasters, and the Mand'alor soon ordered the mission to be aborted the mission and a retreat initiated. Montross refused to comply, unwilling to tarnish their record with a retreat, and pressed forward, only to be caught in a grenade explosion that left him wounded and in need of rescue himself. Mereel provided that rescue, eliminating at least three enemy Kordan before angrily reiterating his orders for a retreat. Simultaneously, Jango and his team stumbled upon a Death Watch ambush, and Vizsla arrived on the battlefield in a four-wheeled tank. Mereel reprimanded Montross's recklessness as the cause of their predicament, vowing to eject him from their mercenary crew if they survived the battle. Vizsla was determined to prevent their survival, launching a wrist rocket that injured Mereel, although Montross evaded it using his jetpack. As Vizsla opened fire with a laser cannon, Mereel called to Montross for an airlift. However, Montross had decided that he would no longer obey Mereel's orders and abandoned the Mand'alor, leaving him to die alone on the battlefield while he returned to the other Mandalorians. Mereel soon succumbed to fire from Vizsla's tank, despite Jango's desperate attempts to reach him. Mereel died from his injuries in Jango's arms.
Upon his return to camp, Montross attempted to seize leadership of the Mandalorians, falsely claiming that Mereel was dead and that Jango had perished trying to save him. However, Jango returned to the camp, carrying his adopted father's body and exposing Montross as a liar and a traitor. As Montross fled into exile, Jango succeeded Mereel as the new Mand'alor, as the other Mandalorians believed Mereel would have wanted, having previously selected Jango as his successor. Jango later avenged Mereel by killing Vizsla on Corellia and subsequently Montross on one of the moons of Bogden.
Following Mereel's death, Jango inherited Mereel's personal starship, an AIAT/i transport which he renamed Jaster's Legacy in Mereel's honor, cherishing it for years to come. Jango also adopted Jaster Mereel's name as a pseudonym during his life and career as a bounty hunter, as a tribute to his adopted father. Jango's cloned son, Boba Fett, would do the same during his own career as a bounty hunter, following his father's death at the hands of Jedi Knight Mace Windu in the First Battle of Geonosis, as well as during his brief tenure as a Journeyman Protector on Concord Dawn. Unknowingly, Boba followed in his grandfather's footsteps, becoming a Journeyman Protector only to be exiled from Concord Dawn after killing a superior officer. In Boba's case, however, the officer in question was a man named Lenovar, whom he had greatly respected, and whom Boba killed after Lenovar raped Boba's wife, Sintas Vel. The use of Mereel's name by both Jango and Boba contributed to rumors that Mereel was still alive and active years after his actual death, and some even believed the opposite—that "Boba Fett" was actually an alias used by Mereel. Boba also recovered Mereel's armor and wore Mereel's distinctive sigil on his armor's breastplate. The knowledge of the circumstances surrounding Mereel's betrayal and death kept Boba wary for years afterward, even in the presence of allies.
One of the Grand Army of the Republic's Null-class Advanced Recon Commandos, a clone trooper designated Null-7, was given the name "Mereel" by the Mandalorian Cuy'val Dar training sergeant, Kal Skirata. Following the Clone Wars and his desertion from the Republic's Grand Army, the good-humored ARC lieutenant went on to become a Mandalorian, just like his namesake.

Jaster Mereel possessed strong morals and a rigid ethical code. Highly esteemed as one of Concord Dawn's Journeyman Protectors, Mereel could not tolerate the corruption of his superior officer and killed him, an act that led him to a new life as a Mandalorian. An exceptional soldier, Mereel's combat skills earned him the respect of his Mandalorian peers. During a period when many Mandalorians had descended into disreputable brigandage, Mereel's strong ethics drove him to become Mand'alor and institute a reform that would change the way the Mandalorians conducted themselves for years to come. Mereel's moral code also prevented him from standing idly by when a young Jango Fett was threatened by the Death Watch, sacrificing his cover to rescue Jango from the more numerous Death Watchmen. Mereel was a confident but pragmatic commander, proud of the skill he and his troops possessed, but unwilling to let that pride influence his decisions, such as whether to retreat in battle. On a personal level, Mereel was a direct speaker who spoke plainly and was unafraid of bluntly assigning blame where he felt it was due. At the same time, he cared for his comrades and especially for Jango Fett, whom Mereel took in when the boy became an orphan and raised like a son until his untimely death. Mereel was also proud to be a mercenary and believed it to be a noble profession worthy of respect.
Jaster Mereel wore traditional Mandalorian armor of a metallic-gray color, with red shoulder plates and red trim around the visor of his rangefinder-equipped helmet. On the left shoulder, Mereel bore the custom Mythosaur skull-inspired insignia of the True Mandalorians, and his chest plates were adorned with a yellow-painted, four-sided diamond. Mereel also wore a red cape, heavy combat boots, and a brown multi-pouched utility belt with his armor, but did not wear or use a jetpack in the field. During his life as a Mandalorian soldier, Mereel utilized a short-barreled blaster rifle that supported the attachment of a combat knife beneath the muzzle, in addition to two blaster pistols which he kept in two holsters around his thigh plates.
Jaster Mereel was originally the true identity of Boba Fett in the novella "The Last One Standing: The Tale of Boba Fett," a detail that was reiterated in Fett's entry in The Essential Guide to Characters. However, with the release of Star Wars: Episode II Attack of the Clones in 2002, its portrayal of Fett's childhood on Kamino as a clone of Jango Fett rendered the previous backstory non-canon. Nevertheless, the four-part comic Jango Fett: Open Seasons retconned the character of Jaster Mereel, establishing him as a distinct individual, the mentor of Fett's father, Jango. It was also revealed that both Jango and Boba Fett occasionally used Mereel's name as an alias, thus salvaging the story from being apocryphal. Daniel Keys Moran, the author of "The Last One Standing," was unaware of the retcons surrounding Jaster Mereel until a 2018 interview, as he has not closely followed the franchise's more recent developments.
Beyond his appearance in Jango Fett: Open Seasons, Mereel was referenced again in Karen Traviss' Star Wars novel Republic Commando: Triple Zero, where Null ARC trooper N-7 was named Mereel, after Jaster. This clone, Mereel, appeared in every subsequent installment of the Republic and Imperial Commando series. Traviss' fourth Republic Commando novel, titled Order 66, featured a quote from Jaster Mereel as the opening for the novel's tenth chapter, and the series' next installment, Imperial Commando: 501st, included several references to Jaster Mereel and the events depicted in Open Seasons.