Derek Fossen was the father of Ben Fossen, and the man responsible for rampaging through Chicago, greatly injuring and killing those in his way, with his primary target being Dr. Mark Greene. He was portrayed by Ted Marcoux.
Fossen first appeared in "Where the Heart Is", where it was revealed that he was responsible for abusing his young son, Ben, prompting Dr. Mark Greene to put Ben in protective custody, with Ben later being put in foster care.
Having lost his son, Fossen was ultimately driven over the edge and, seeking revenge against Greene, he goes on a shooting rampage throughout Chicago, targeting anyone who got in his way, including some children residing at a foster care facility. He later attempted to strike Greene's home, presumably intending to kill Greene's wife, Dr. Elizabeth Corday, but was stopped by a civilian who shot Fossen.
Badly injured, Fossen was subsequently transported to County General Hospital where Greene and his own ER colleagues worked on stabilizing Fossen.
Later, while in the lift heading for the OR, Fossen's vitals dropped but Greene, enraged over Fossen's attempts to destroy his family and over the wave of destruction Fossen had unleashed in his quest for revenge, shocked the air with the paddles instead of shocking Fossen, which ultimately led to Fossen dying a slow and painful death.
In "Four Corners", due to Fossen's death, an M&M conference was called with the senior staff of various departments in attendance.
Fossen was a dominating, controlling figure, easily prone to anger and who wanted his son kept with him at all costs.
When he lost his son, he ultimately slipped into a state of rage which in turn drove him over the edge. This caused him to begin attacking everyone in sight while sparing no remorse or mercy for his crimes, suggesting that he was probably a psychopath.
In the ER, while being treated after being shot, Fossen blamed Greene for destroying his family, implying that Fossen had a very strong grudge against Greene and resented him for what he had done.