Boba Fett, one of the most mysterious characters in the Star Wars franchise with one of the largest fanbases, lived on in the minds of imaginative fans long after he was presumably eaten by the Sarlacc in Return of the Jedi. After his death, fans only had the novels, video games, and comic books of the Star Wars Legends catalog to follow the adventures of their favorite bounty hunter, never knowing that decades later, The Mandalorian would bring him blazing back into their lives.
The library of content contained in Star Wars Legends (media adjacent to the Skywalker saga and created for the franchise before it was purchased by Disney) included hundreds of narratives fleshing out the Star Wars Universe, and there was no character more intriguing than the mysterious Boba Fett. Given the fact that so many different creators brought their own touch to the mercenary, he differed considerably from the stonecold killer played by Jeremy Bulloch and Temuera Morrison in canon
Physical Appearance
When Jango Fett supplied the genetic material necessary for the Kamino cloners to create a clone army for the Republic, he requested an unaltered clone he could raise as a son, so canonically Boba Fett looks exactly like his bounty hunter father.
Prior to George Lucas's Prequel Trilogy, Boba Fett wasn't a clone but a man named Jaster Mereel (from The Last One Standing: The Tale of Boba Fett, and not to be confused with the name of Jango Fett's mentor), with pale skin, black hair, and a large scar on his face, courtesy of a young Han Solo. He worked as a member of Corellian Security and had run-ins with the young smuggler, slowly building a foundation of resentment and seething hatred.
Clan Fett
The current canon doesn't extend very far into Jango Fett's past, thus Boba Fett doesn't appear to know much about his family history. According to Boba, Jango's parents were killed sometime before the Clone Wars and as a foundling he was adopted by a Mandalorian clan, giving him some heritage to be tenuously connected to.
In Legends, Fett came from a long line of Mandalorian warriors, their clan name is taken from the Mandalorian word for "farmer" ("vhett"). Fett's ancient ancestor Cassus Fett was introduced in The Knight of the Old Republic paraphernalia, thousands of years before the Clone Wars. Later came Vorten Fett, Khomo Fett, and Boba himself contributed to the clan by marrying Sintas Vel and fathering Ailyn Vel, and grandfathering Mirta Gav.
The Death Of His Father
In Star Wars: Episode II - Attack of the Clones, fans are introduced to bounty hunter Jango Fett and his son Boba. Jango has been recruited by a member of the Jedi Order to be the template for a clone army, an army which he fights against while on the Separatist's payroll. When clone troopers are sent to rescue Jedi on Geonosis, their general Master Mace Windu ends up killing Jango right in front of Boba.
If the Boba Fett from Legends was depicted by authors prior to the release of Star Wars: Episode I- The Phantom Menace, he bore no such grudge against Mace Windu (who also didn't exist yet) or the Jedi in general for robbing him of his father. Much of his family was clouded in mystery to add to the character's mystique.
Conversation Skills
Boba Fett has always had a monosyllabic screen presence, with the bounty hunter's choice to be economic with his words adding to the aura of mystery that surrounds him. In all his appearances since The Empire Strikes Back, he doesn't say any more than he has to, often not volunteering information unless asked directly by characters like Din Djarin or Bo-Katan.
In Star Wars Legends he could be downright chatty, such as in A Barve Like That from the anthology Tales From Jabba's Palace, where he has a pretty snappy report with Princess Leia. After she's been exposed trying to free Han Solo from carbonite, she's offered to Fett for companionship the night before Luke's arrival, and the two discuss everything from morals, to honor, to loyalty.
Personality
Stoic. Reserved. Impregnable. These are words that have been used to describe the galaxy's most feared bounty hunter. Fett communicates more effectively by what he doesn't say, using the anonymity his armor affords him to be intimidating. His demeanor and body language throughout the course of his life as a career killer have been enough to guarantee he is not to be trifled with.
While no one would describe Boba Fett as congenial, in The Bounty Hunter Tales novels, he grew close to characters like Connor Freeman, and would even share a drink with a peer like Dengar. The most salient aspect of the Legends media was that fans were in Fett's head continuously, which gave them an idea of the sort of man he was.
His Complex Connection To The Mandalorians
In The Mandalorian's second season, Boba Fett informs Mando that his father Jango Fett was a Mandalorian who fought in the Mandalorian Civil War (not to be confused with the one that occurred in The Clone Wars after he died). Jango is referred to as a Mandalorian foundling, and by giving Boba his armor, Boba has a right to wear it despite not being part of the Mandalorian culture.
In Legends, the Mandalorian Civil War was known as The Great Clan Wars instead, and it occurred decades before the events of Star Wars: Episode I- The Phantom Menace. In some material, Jango wore beskar, spoke Mando'a, and even became the leader of Mandalore himself, raising his son to be part of Mandalorian culture. In other material, Boba Fett acquired the armor of a Mandalorian in his own adventures and had no connection to Mandalore.
His Politics
Canonically Boba Fett describes himself much like his father - "a simple man just trying to make his way in the galaxy". He's principled, but like most bounty hunters, will extend his services to the highest bidder no matter their political affiliation. He's not shown to be particularly political, nor having much of an opinion one way or the other about the Alliance or the Empire in terms of the Galactic Civil War.
From comics like Death, Lies, and Treachery and Enemy of the Empire to Legends stories like A Barve Like That, Boba Fett expressed what could amount to political opinions. In one instance, Leia tried to recruit him to the Rebellion, but he refused on the grounds that the Rebels were on the losing side, and were causing chaos where the Empire was providing stability, appearing at the very least to be both conservative and pro-authoritarianism.
Love Interest
Boba Fett is a man uninterested in the pursuit of forming a romantic interest in anyone. Similar to Din Djarin, he seems to find that any emotional attachment is a liability in his line of work. He's never shown to have a love interest as part of the present canon, though he's shown flirting with one dancer at Jabba's palace in Return of the Jedi.
In Legends, he also shied away from love for the most part, but some authors provided him with a partner. In Legacy of the Force: Bloodlines, an aging Fett contemplated his legacy and looked back on his marriage to Sintas Vel, a Kiffar bounty hunter whom he settled down with on Concord Dawn, and with whom he had a daughter who later became estranged.
The Years After Escaping The Sarlacc
The Mandalorian makes it clear that Fett has been languishing on Tatooine for years following his emergence from the Pit of Carkoon. Without the aid of his Mandalorian armor, he's been made far more vulnerable, and he's been forced to survive by any means necessary.
This was in stark contrast to Legends where he spent years after surviving the Sarlacc either pursuing Han Solo (who was going through a mid-life crisis) or shaking the very foundation of the Bounty Hunter Guild, its members scrambling to pick up the contracts left in the wake of Fett's supposed death.
His Adventures
If fans based Boba Fett's reputation for being the most feared bounty hunter in the galaxy on what they saw of him canonically, they'd be disappointed. The Mandalorian is the only live-action incarnation of Fett that shows him actually being worthy of his legacy, clobbering stormtroopers with little more than a gaffi stick, and the scenes come a decade after Fett did any real bounty hunting.
In Legends, Fett's reputation was secured with how much of it was earned. He worked for Darth Vader on several separate occasions (even going so far as to battle the Dark Lord of the Sith himself), stood up to Prince Xizor of the Black Sun Syndicate, teamed up with infamous associates Bossk and Dengar, and had many more run-ins with his nemesis, Han Solo.