In the 1973 TV movie, he is played by the late Kirk Douglas.
He created a chemical formula designed to rid him of his negative impulses and thoughts. Instead, the formula condensed his darker nature into a monstrous being known as Edward Hyde. Now, whenever Jekyll drinks the formula, he transforms into the monstrous Hyde, with different versions of the story showing how Hyde turns back into Jekyll (sometimes the formula is on a time limit, sometimes it happens when Hyde calms down, sometimes there's an antidote for the formula).
Unlike Hyde, Jekyll is kind, gentle, friendly, polite and noble, which is why he is constantly emotionally tortured by the crimes that Hyde commits.
In various crossover media, Jekyll proves to be a reliable ally to the heroes, as opposed to the antagonistic Hyde. In the 2017 film The Mummy, Jekyll is the head of Prodigium, an organization dedicated to protecting the world from monsters (with the ironic twist that Hyde is himself one of the monsters that Jekyll tries to protect the world from). In the film The League of Extraordinary Gentlemen, Jekyll is willing to use his scientific knowledge to help the League and is often reluctant to let Hyde help the League, and is horrified when he finds out that Professor Moriarty reverse-engineered the formula, declaring "I will not let my evil infect the world!".