When Declan was young, his mother Eva Brady adopted a baby she found in the hospital. Although Eva treated Declan and Finn equally, Declan harbored hatred for his new brother as they grow up. As Finn became a famous medical doctor, Declan lost his way and became a psychopath. Declan started to cause trouble in the family, and he even started borrowing money from Finn. Overtime Declan became a calculating serial killer. One of his plans was to frame Finn for his murders while manipulating his wife for his own gains. Their mother soon died and Declan mummified her remains in his house.
Ironically, Declan became a huge fan of Finn's uncle (who was unbeknowst to him) John Constantine (he even had the same coat and cigars as John). Declan finally got his chance to meet Constantine when the magus arrived in Ireland looking for his lost nephew. Unfortunately, Declan found out that the nephew was actually his brother Finn, and this increased his hatred for him even further. When John learned of the killings that was happening there, Declan managed to make it look like Finn did it using magic. At first, Constantine thought that it was his nephew who was the killer, but after learning about Declan, he then sets off to find him. He managed to track Declan down in his own house. While the latter got the drop off Constantine, the magus fought back by reanimating Declan's mother, who then kills him.