Mariano was a car washer and cashier at the A1A Car Wash.
After Gus Fring's murder, Skyler White became withdrawn and despondent. Lying at home in bed, she said that Mariano could open the car wash in her place ("Madrigal").
When talking with Hank Schrader about Skyler's mental state, Walt said that Mariano keeps an eye on her when he's not around at the carwash ("Dead Freight").
Walt greeted several employees by name on arrival at the car wash one morning including Mariano, Enrique & Luis ("Blood Money").
Mariano was managing the cash register at the car wash when Walter called and then later rushed in looking for Skyler after she had gone to meet Hank Schrader ("Buried").