Cave Johnson is the founder and CEO of Aperture Science, the research company whose labs serve as the setting of Portal and Portal 2. He is not seen or heard in the original game, but his existence was fleshed out in external materials that described the history behind Aperture Science and the road that led to the development of the portal gun technology and GLaDOS. In Portal 2, only his voice is heard as the player runs through the older, deeper test chambers. The character is voiced by actor J.K. Simmons.
Leading up to the release of Portal 2, Johnson provided the narration for humorous trailers patterned after Aperture Science industrial films designed to highlight the fruits of the company's research and their line of products.
Cave Johnson plays the role as the narrator (through pre-recorded messages) for Chapters 5 and 6 of Portal 2, where Chell finds herself in the original foundation for Aperture. As she makes her way through the facility, a brief history of Aperture and Cave Johnson is described. Chell (and later GLaDOS) learn of Caroline, Johnson's secretary, friend, and possibly, love interest.
In the beginning, Cave is shown as a cheery "All-American" CEO, who doesn't seem to know too much about what his company actually does, just knowing that it's "for science" and referring to his scientists as "lab boys." Aperture is originally well-funded, riding off their successes as a shower curtain company, reflecting Cave's optimism. As Chell progresses to the later built areas, Cave expresses anger at Black Mesa for stealing their funding. This is also where the dry docks for the Borealis is found, which was most likely a result of Aperture's desperate attempts to create innovative products.
Aperture eventually goes bankrupt, driving Cave to desperation. They are experimenting with a new gel, created from moon rocks, that provides a surface for portals to be made on. Cave demands that they proceed with grinding up the moon rocks, and immediately learns that ground-up moon rocks are pure poison. Although the gel works, Cave is severely internally damaged and begins to slowly die. During this time, he desperately asks for Caroline's comfort as he demands that his scientists work on a project to make him live forever by "pouring him into a computer." He eventually realizes his mortality, knowing the project will never be finished, and announces that Caroline would be put in charge when he dies. Honoring his legacy, the scientists eventually infuse Caroline with GLaDOS so she could finally "live forever," fulfilling Cave's wishes.