Krazy-8 portrayed the first primary antagonist in Breaking Bad, but further details about his backstory weren't delved into until his appearance in Better Call Saul. The character, whose real name was Domingo Gallardo Molina, was played by Maximino Arciniega. Introduced in the Breaking Bad pilot, Krazy-8 was a DEA informant who had a major role in shaping Walter White (Bryan Cranston) and Jesse Pinkman (Aaron Paul) as ruthless figures in the meth trade.
After threats from Krazy-8, Walt invited him and his cousin, Emilio, to learn the meth recipe. Krazy-8 and Emilio forced Walt by gunpoint into the RV but instead of mixing the recipe, the expert chemist concocted a phosphine gas explosion. The gas killed Emilio, but Krazy-8 lived, so Walt had no choice but to lock him in Jesse's basement, working up the courage to kill him. Eventually using the bike lock chained to the pole, Walt strangled Krazy-8 and killed him before his body was then disintegrated by hydrofluoric acid, making him Walt's first victim on Breaking Bad. The path that led Krazy-8 to become a drug distributor was revealed in the prequel series.
Arciniega reprised his portrayal as Krazy-8 starting in Better Call Saul season 2. The recurring character was first shown working for Tuco Salamanca after he delivered the man cash at a Mexican restaurant. In Breaking Bad, Krazy-8 revealed tidbits of his early life including that he had a Business Administration from the University of New Mexico. He wanted to study music but his father forced him into working for the family business, Tampico Furniture. The Krazy-8 we saw in Better Call Saul still worked at the furniture store since he was often seen wearing the Tampico polo shirt.
Following Tuco's arrest, Krazy-8 kept working for the drug trade under Nacho Varga and Tuco's uncle, Hector Salamanca. In Better Call Saul season 3, Krazy-8 delivered a cash payment to Nacho but the payment was light. Nacho was going to let it slide but after Hector's reaction, Krazy-8 was severely beaten for the payment mishap. This event caused Krazy-8 to deliver his payments in a more timely fashion.
Through Better Call Saul season 4, Nacho took over a larger role in the Salamanca drug trade. He also made it a point to train Krazy-8 in using violence if his buyers aren't paying on time. This tougher reputation made him a more confident seller which will evidently lead him into a more independent distributor by the time he meets Jesse in Breaking Bad. Krazy-8's role as a DEA informant will most likely be revealed in Better Call Saul season 5, which is set to air in 2020.