Leonel and Marco Salamanca, better known as "The Cousins," were a pair of antagonists in Breaking Bad, but their deeper connection to Gus Fring wasn't explained until Better Call Saul. The well-dressed twin brothers who barely spoke, played by Daniel and Luis Moncada, respectively, were introduced in Breaking Bad season 2 as hitmen for the Juárez Cartel. It was revealed via a flashback that the Cousins helped their boss, Juan Bolsa, kill the drug runner Tortuga for being a DEA informant.
The Cousins were still in Mexico at the start of Breaking Bad season 3. They learned of the death of their cousin, Tuco Salamanca, so they targeted Heisenberg without knowing his identity. After crossing into the States, the bothers paid a visit to their uncle, Hector Salamanca, who used a Ouija board to unveil Walter White as the man they were looking for. They embark on an assassination attempt but are stopped by Gus who controlled the drug trafficking in the area. He claimed that Walt was still too valuable so the Cousins would have to wait. Leonel and Marco then learn that Hank Schrader was the one who killed Tuco so they ambushed the DEA agent in the parking lot. Hank viciously fought back, killing Marco in the process. Leonel was taken to the hospital but later died from cardiac arrest due to a lethal injection by Mike Ehrmantraut at the request of Gus. The path that led them to this point was later revealed in the prequel series.
The Salamanca family bond was already explained prior to the debut of Better Call Saul. The family's loyalty was put on display multiple times considering how many Salamanca men set out to avenge the death of another family member. The Moncada brothers reprised their role for Better Call Saul season 2 and gave more evidence to the strong Salamanca bond. After Mike was approached by Hector to clear Tuco's assault charges, the Cousins were sent as intimidation. The twins were seen on the rooftop keeping an eye on Mike's granddaughter, Kaylee. The threat to her life forced Mike into taking Hector's deal. When one of the truck drivers was ambushed, Hector used the Cousins to execute the man for losing his drug money.
The Cousins returned in Better Call Saul season 4 after Hector suffered from a stroke. When Nacho and Arturo were attacked in the desert, the Cousins came to their colleagues' rescue. When they arrived, they found that Arturo was killed and Nacho had severe injuries. The attack was set up by Gus, Hector's biggest rival, with the help of Nacho, but the ambush was pinned on the Espinosa Gang. The Cousins then set out to wipe out the entire gang, which they did successfully. Shortly thereafter, the pair traveled back to Mexico so they could hide out for a while.
Getting the Cousins out of the country was a major advantage in Gus' takeover of the territory. The Salamanca family lost another big piece of the puzzle, at least for a brief time. It's assumed that the Cousins stayed in Mexico until they dangerously crossed the border in Breaking Bad. It's still possible that the hitmen could appear in Better Call Saul season 5 which is set to air in 2020.