- Walter: "I'm the cook. I'm the man who killed Gus Fring."
- Declan: "Bullshit. Cartel got Fring."
- Walter: "You sure? That's right. Now. Say my name."
- Declan: "Heisenberg."
- Walter: "You're goddamn right."
- —Walt and Declan.[src]
Declan was a meth distributor based in Phoenix, Arizona and was a competitor to Gustavo Fring's former drug empire. The stories of the notorious meth cook 'Heisenberg' have reached him within the past year. He was shown to be aided by bodyguards.
Mike Ehrmantraut first contacts him through the phone to set up a meeting in the desert. When he, Jesse, and Mike all come face to face, they attempt to strike a deal that would give him Jesse and Mike's shares of the recently stolen methylamine; however, it becomes compromised when he refuses to buy just 2/3 of the lot, explaining that the steep price is also paying to have his competitor's Blue Sky off the market. ("Buyout")
Days later they meet again, but with Walter White in charge of the meeting. He convinces Declan that everyone could make more money if Walt cooked the methylamine to produce 99.1% meth while the best Declan could make was 70%. Before Declan can agree, Walt demands that Declan "say my name" to which he responds "Heisenberg". Declan then makes the deal with Walt, and gives $15 million to him, Jesse and Mike. ("Say My Name")
Following Jesse's retirement from the meth business, Walt continues to work with Declan, by having his assistant cook Todd Alquist make drop-offs of meth to Declan's man in Arizona. ("Gliding Over All")
After Heisenberg's retirement, Todd takes over as the primary cook for Declan and his partner, Lydia Rodarte-Quayle. At some point, Declan fires Todd and resumes using his initial meth cook. Later, Lydia meets with Declan and his crew at their hidden meth lab in the Arizona desert and tells them their low quality methamphetamine isn't good enough to sell in the Czech Republic and they are leaving $50 million on the table. Declan refuses to clean up his operation or hire Todd back. At this point, Declan and his men are ambushed by Jack's White Supremacist Gang, acting on Lydia's orders. A wounded Declan tries to crawl away from the carnage, but he is shot dead by the gang's leader, Jack. ("Buried")