As a prequel to Breaking Bad, Better Call Saul is about many things, the most important being Jimmy McGill's gradual transformation into Saul Goodman. The midseason finale of the final season solidified this transformation.
While fans wait for Better Call Saul to return and air its final episodes, it is the perfect time to look back at how Jimmy McGill became the Saul Goodman character that fans know from Breaking Bad. Even if it wasn't always intentional, through their actions, many characters contributed to the idealistic Jimmy McGill becoming the unscrupulous, sleazy criminal lawyer Saul Goodman.
Marco Pasternak
Saul Goodman wouldn't exist without Slippin' Jimmy. This was the nickname Jimmy had when he ran scams in Cicero, Illinois, many of which involved Jimmy's good friend Marco Pasternak. Through working with Marco, Jimmy learned the art and the thrill of a good scam, all of which would be integral to his life and identity as Saul Goodman.
When Jimmy returned to Cicero years later, he and Marco reunited and rekindled their scams, and rediscovered the joy of conning and outsmarting people for money. Before suffering from a heart attack and dying during their final scam, Marco told Jimmy the past week they shared together was the best of his life. Marco's final words and what Jimmy rediscovered with him served as a reminder to live life on his own , even if those are dishonest.
Tuco Salamanca
Tuco Salamanca was the first Better Call Saul character who connected Jimmy McGill to the world of the cartel. Enraged by a con that Jimmy and two skater brothers tried to pull on his grandmother, Tuco took Jimmy and the brothers into the desert. Jimmy is able to save himself and the twins--minus some broken legs for the twins--through some of his signature persuasive talking.
Even though this was the last time that Jimmy and Tuco crossed paths, this incident connected Jimmy to Nacho Varga, and Jimmy's connection to Nacho connected him to Lalo Salamanca. If not for Tuco, Jimmy might never have become the criminal lawyer Saul Goodman who was a friend of the cartel.
Nacho Varga
Tuco is the one who brought Jimmy into the world of the cartel, but Nacho is the one who made sure Jimmy stayed there. After the incident with Tuco, Nacho realized how invaluable Jimmy could be and forces Jimmy to be his lawyer when he is in trouble. Jimmy comes through and gets Nacho out of police custody.
Knowing that Jimmy already has a connection to the cartel and has helped someone in the Salamanca organization--Nacho--get out of police custody before, Nacho seeks out Jimmy to help when "Krazy-8" Domingo Molina is arrested. At this point, Jimmy is already practicing law under the name Saul Goodman. This solidifies that Saul Goodman doesn't just defend small-time criminals. It also connects Saul to the dangerous Lalo Salamanca, as he is in charge of the Salamanca operations in Albuquerque at this time.
Lalo Salamanca
When Lalo Salamanca is charged for murder, he has Saul Goodman represent him in court, seeing as Saul already successfully represented individuals like Nacho Varga and "Krazy-8" in the Salamanca organization. Lalo insists that Jimmy needs to convince the judge to grant bail. Jimmy goes to more dishonest and corrupt lengths to make this happen, including hiring actors to play the family of the man Lalo is pretending to be so he can gain sympathy in court and prove that Lalo has community ties to Albuquerque.
Jimmy gets into the cartel world even deeper when he is sent to retrieve Lalo's bail money in the desert and gets in the crosshairs of cartel rivalries, but he is ultimately successful in getting Lalo the money, which gets Lalo out of jail. Criminals of all kinds soon begin flocking to Jimmy for business as they know him as "Salamanca's guy." The perception is that if Saul Goodman was trusted to represent Lalo Salamanca and successfully got him out on bail, then he can do whatever it takes to get the best deal for any criminal.
Mike Ehrmantraut
Mike and Jimmy's relationship began in a mundane way, with Mike being a parking lot attendant at the Albuquerque courthouse and Jimmy being a lawyer who usually didn't have enough stickers to park there. Their relationship became much more complex when Mike hired Jimmy to be his lawyer, with both of them doing dishonest things to help each other.
From there, they gradually become more absorbed into the world of the cartel, with both of them relying on each other's unique skillsets when needed. Mike even saves Jimmy's life when Jimmy gets in over his head with cartel business in one of Better Call Saul's best episodes.
Cliff Main
For years, Jimmy aspired to work alongside his brother at Hamlin, Hamlin & McGill (HHM). This didn't work due to Chuck McGill refusing to allow Jimmy to work at HHM. Instead, Jimmy's first corporate job at a law firm was with Davis & Main.
Even though Cliff Main was a generous and fair boss, Jimmy soon realized he didn't like working in a corporate environment where he couldn't take ownership of his work and bend the rules, even when it was better for the clients. Cliff Main worked hard to try and make Davis and Main work for Jimmy, but all it did was show Jimmy that he could never be a corporate lawyer. Jimmy needed to be his own boss and do things his own way.
Howard Hamlin
Howard Hamlin liked Jimmy McGill, even giving him the nickname "Charlie Hustle" since he saw Jimmy as a real go-getter. Whether it was getting Jimmy a job at the Davis & Main law firm or offering Jimmy to become a partner at HHM after Chuck's death, Howard tried to help Jimmy.
Nevertheless, Jimmy always resented Howard. From Howard's relationship with Chuck to the many advantages Howard had in life that Jimmy did not have, Jimmy hated Howard and made him his punching bag. Howard was an easy target for Jimmy's many frustrations. By making Howard a frequent mark of his, Jimmy learned how to better con and outsmart other lawyers. He learned how good it felt and couldn't help but work with Kim Wexler to destroy Howard's life. Through the ways that he perceived Howard wronging him, all of this felt justified to Jimmy, leading to a tragically perfect ending for Howard.
Chuck McGill
Jimmy spent most of his life trying to prove himself worthy to his brother Chuck. When it became clear that Chuck would never see Jimmy as a real lawyer and would always look down on him, Jimmy knew he would have to make his own way. Jimmy could now embrace the manipulative, sneaky, unscrupulous way of practicing the law that was natural to him without feeling guilty and trying to be a person that Chuck would respect.
In order to outsmart his brother, Jimmy tossed all the rules out the window, and he never went back to them again. After Chuck's death, instead of confronting his trauma and any guilt he might have, Jimmy formed his Saul Goodman persona and sunk deeper and deeper into it, eager to have an identity that wouldn't have to live in Chuck's shadow.
Kim Wexler
Jimmy loves Kim more than anyone in the world. She loves him just as fiercely and has always seen the good in Jimmy, even when no one else could. Jimmy usually kept his more immoral and manipulative dealings separate from his relationship with Kim.
However, when Kim made it clear that she wanted Jimmy to help her disgrace Howard Hamlin beyond repair, this led to Jimmy doubling down on his worst tendencies. There was no longer separation between Jimmy McGill and Saul Goodman. They became one as Jimmy pulled out of all the stops to con and disgrace Howard, and unlike in the past, he didn't feel guilty about it, as it was what Kim wanted and they were actively working together to make it happen.