This isn't the first time one of you Bulldogs has come in here, looking to 'cause trouble.
~ Mustang to Archie.
Mustang is a very minor character but also the secondary antagonist in the first season of Riverdale. He was a member of the Southside Serpents and one of main contributors to Jason Blossom's torture and murder.
Very little is known of Mustang's life. He resided on the south side of town in City Centre Motor Hotel, and sometime prior to the start of the series he joined the Southside Serpent, a dangerous biker gang of criminals and thugs.
Serpents vs Bulldogs
He was first seen in the Serpent's usual hangout, a local pub called Whyte Wyrm when he was confronted by Archie Andrews after the latter mistook him for another serpent he saw vandalizing his father's construction site and attacking one of his friends. Mustang was about to attack Archie however, FP Jones intervened and stopped the confrontation between the two.
Torturing Jason Blossom
He was next mentioned when FP's right hand Joaquin revealed that Mustang had involvement with cleaning up the evidence of Jason Blossom's body and took Archie and his friends to talk to him about it. When they arrived at the hotel he was staying at however they went into the bathroom only to find Mustang's dead body awaiting them in the bath. The police then suspected that Mustang died of drug overdose and did indeed have more involvement with the Jason Blossom case than he was letting on.
After Betty, Jughead and Kevin found a duffle bag they took it to Archie's house and opened it to find a USB drive inside of Jason's jacket. They put the drive inside of Archie's laptop and much to their horror saw video footage of Jason's murder where Mustang was seen torturing and taunting him awaiting Clifford Blossom to arrive and kill Jason.
In the season one finale, it was revealed that Clifford killed Mustang and staged it as suicide.
Mustang was a wrathful, sadistic man as well as a thug. Not much more is known about him however due to his minor role however he apparently did show a level of respect for his fellow serpents.