Jim Ellison was born with hyperactive senses. During his childhood, he had a friend, Bud who helped him with said senses. Unfortunately, Bud was murdered at some point, one of many cases of trauma in his childhood. His mother also left, leaving him to live with his extremely wealthy father, William Ellison, and his brother, Steven Ellison. Jim's relationship with both characters is notably strained, and leads to both the repression of his senses and his joining the Army.
While in the Army, Jim achieves the rank of Captain in the US Army Rangers, working covert ops. His last mission takes him and his team of seven to Peru to stop a drug-smuggling campaign. Unfortunately, his ranking officer was corrupt and set him and his men up for failure. His helicopter crashes, killing his seven men and leaving him wounded. He is taken in by the Chopec, an elusive and secretive tribe in Peru, and learns to control his senses for the tribe's betterment under the tutelage of Incacha, the tribe's shaman, who calls Jim "Enqueri" ("Sentinel"). The following year, when a satellite photo shows seven tombstones at the helicopter's crash site instead of just a pile of scrap metal, the US Army sends in a team to find the sole survivor of the crash and bring him back home.
When Jim returns to Cascade, Washington, he represses the memories of his time in Peru. Nevertheless, his instincts as a Sentinel remain, leading him to "serve and protect" his "tribe," the residents of Cascade, by joining the police force. First assigned to Vice, then eventually assigned to Major Crime, he is mentored by Jack Pendergast, who helps Jim tame his temper and his rebellious attitude. Jim later marries Carolyn Plummer, another detective on the force. Jack then dissapears under mysterious circumstances. Carolyn and Jim divorce.
Shortly afterwards, Jim is sent on a solitary stakeout for a week in a remote woodland location, during which his senses start to re-emerge, primarily his sight and hearing. After his return to the city, his enhanced senses fully emerge, leaving him feeling disoriented, 'stressed out' and drugged. He can see, hear, and smell things that no other person could, he can only handle the blandest of foods, and any barely-coarse materials leave him wishing to claw his skin off. He checks himself into a hospital, where he meets Blair Sandburg posing as Doctor McCoy (as it says on the stolen lab coat he wears) or Doctor McKay (as he pronounces it).
During the series Jim falls in love with a handful of women but most of them end up dying in his arms. Several of Jim's old friends from the Army and police force also end up dead during the course of the show.
As a Sentinel Jim has several powers:
- All 5 senses are strongly enhanced
- Able to communicate with ghosts
- Has a spirit animal, a black jaguar
- Receives visions that guide him in the choices he makes and sometimes predict the future (Jim had a vision that showed Blair's death before Alex killed him)
- Used the power of his animal spirit to bring Blair back from the dead