Declan was a meth distributor based in Phoenix, Arizona and was a competitor to Gustavo Fring's former drug empire and an associate of Mike Ehrmantraut. He is initially contacted by Mike to purchase methylamine in order to buy out his and Jesse Pinkman's share of their partnership in Walter White's drug empire. However Walt blocks the deal and instead makes Declan and his men his distributors by buying out Mike's partnership for $5 million.
Following Walt's retirement, Declan employs Todd Alquist to cook Walt's meth however he fails due to inexperience and Declan fires him and uses his own cook to produce lower grade meth in Arizona, which still turns a good profit. He and his entire crew would ultimately be killed by Jack Welker on orders from Lydia Rodarte-Quayle for refusing to hire back Todd and improve the quality of their meth to sell to her buyers overseas.
Prior to the events where he is shown on-screen, Mike says that he was a contact from when Mike was working with Gus' meth empire.
Mike 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. ("")
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, buying out Mike's partnership as distributor for $5 million. ("")
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. ("")
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 Welker's 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 Welker. ("")