The main character is always the most important character on a TV show -- except when they aren't. In many shows, the most important character actually died before the story even begins. Despite being dead by the time these series began, these deceased characters are responsible for setting important events in motion.
Aerys Targaryen, "Game of Thrones"
Better known as "The Mad King," Aerys Targaryen was the last member of House Targaryen to rule from the Iron Throne. After going insane, his sworn guard Jamie was driven to kill him during Robert's Rebellion for the good of the kingdom. Father to both Viserys and Daenerys, Aerys is really the character whose actions set all of the plots spinning in "Game of Thrones."
In fact, there are a number of other characters who are mentioned frequently in the series, despite being dead before the show began. These include Lyanna and Rickard Stark, Jon Arryn, and Rhaegar Targaryen. While their actions are important, Aerys is still the one who sets the ball rolling.
Laura Palmer, "Twin Peaks"The FBI would have never had occasion to visit Twin Peaks had it not been for the murder of Laura Palmer. Her tragic death sets the entire series in motion, and her relationships with other people shape their behavior long after her death.
Harry Morgan, "Dexter"By the time the first season of "Dexter" kicks off, Harry Morgan has been dead for quite some time. However, he still manages to appear in nearly every episode as the personification of Dexter's conscience. Harry is arguably the reason that the "Dexter" show can exist: It was Harry who trained his son in the art of vigilante killing. However, viewers eventually learn that Harry regretted teaching his son how to avoid getting caught by the authorities.
Maximino Arciniega, "Breaking Bad"Max is a very minor character who is dead long before the show begins. Originally partners with Gus in the drug trade back in the day, Max's death puts all kinds of events in motion. His death inspired Gus to start a scholarship in Max's name, which Gale eventually wins. Gale, in turn, is how Hank can link Walt to Gus Fring. Max's death leads to both Walt's entrance into the "big time" drug world, as well as his eventual unmasking to the authorities.
Dr. Jonas Venture, Sr., "The Venture Bros.""The Venture Bros." follows the Ventures, a family of super-scientists. At the start of the series, the main members of the Venture clan are Dr. Thaddeus "Rusty" Venture and his twin sons, Hank and Dean. Dr. Venture struggles with panic attacks, performance anxiety, and all kinds of physical and mental health issues brought on from childhood trauma at the hands of his dad, the deceased Jonas Venture, Sr.
All of Rusty's shortcomings are due to his father, who frequently brought him on dangerous "adventures" all over the globe. Without the elder Dr. Venture's influence, Rusty might not be such a pill-popping mess today.