"Charlie" was the nickname given by Phoenix Project scientists to a great white shark they used in their experiments, based on the shark's scientific name Carcharodon carcharias. It was otherwise simply designated "Phoenix." Charlie was given the Phoenix unit, which mutated it into a bipedal, humanoid shark creature that possessed shapeshifting abilities, in particular the power to liquefy itself to ooze beneath doors and through ventilation ducts.
It climbed out of its tank and ate all of the lab's other sea life specimens in the other tanks, and even attacked and killed several lab personnel before being seemingly killed by security. However Charlie didn't die. The Phoenix unit in its genetic structure revived it, and it went on a killing spree, slaughtering and eating all of the Phoenix Project personnel and absorbing their minds into its own, gaining self-awareness from them. Prior to absorbing the minds of humans, Charlie remained a simple-minded predator; only when it began to consume human genes did it start gaining intelligence.
The "trapped" victims remained conscious within Charlie's mind, but only the most strong-willed, such as project leader Dr. Gordon Shelley, were able to assert dominance over Charlie's consciousness and take over the shark's body, using its shapeshifting abilities to transform its body into theirs, but only for short periods of time, because Charlie always regained control. When Paul Latham and his fellow castaways stumbled across the Phoenix Project facilities, Charlie stalked them, eager for fresh new genetic material to eat.
Despite occasional interference from the minds of Shelley, Dr. Carol Soames and security guard Ed Buckley, Charlie was able to absorb first Rochelle, and then, taking her form, seduce and eat her boyfriend, the sadistic Alex Rinaldo. Alex's mind briefly dominated Charlie's, and it was while Charlie's body was under Alex's control that he raped, killed and consumed Chris. Shortly thereafter, as Alex attempted to do the same thing to Linda Warner, Charlie reasserted himself, subsuming Alex's mind and reverting to its original state.
Charlie's downfall however came when it absorbed Chris' heroin addict boyfriend Mark. The shark mutant became addicted to the heroin, and was seemingly killed when Paul and Linda injected most of the remaining heroin into its body. However, the Phoenix unit allowed Charlie to evolve a means of transferring its genetic material to a new body that wasn't addicted to heroin, choosing a hapless Brinkstone helicopter pilot; the Phoenix-infused Charlie genes completely took over the pilot and the original Charlie body, dying from severe heroin withdrawal, was left behind and perished.
In the guise of the pilot, Charlie began flying Paul and Linda to the mainland. Midway through the flight, however, Charlie revealed its true identity, and planned to also spread its genes to them, transforming them into clones of itself. Before it could, Linda, gambling that Charlie was still addicted to heroin, having carried that over into its new body's genetic memory, offered the creature the last of the drugs. When Charlie grabbed for them, Paul opened the helicopter door and kicked Charlie out. It fell into the sea.
Charlie's background, abilities and behavior are much the same as in the novel, except the project is called Proteus instead of Phoenix, and he is seemingly destroyed by fire at the end of the film only to survive and disguise himself as Leonard Brinkstone's helicopter pilot. His deception is not discovered by Alex and Linda and Charlie's current whereabouts are unknown.