In the two games there are various "Ratman dens", where Doug Rattmann has left scribblings and paintings on walls in hidden rooms.
[1] Already skeptical of the computer, the man fled from the gas and kept himself hidden from GLaDOS' view, slowly becoming more insane over an unknown stretch of time.
During events in Portal, he worked behind the scenes to scribble messages and warnings to Chell on the walls, leading her out of the testing chambers and towards GLaDOS.
After watching her defeat the computer, he managed to escape the facility, but returned to assure Chell would be put in indefinite cryogenic storage animation after she was dragged back inside, suffering a serious injury (a shot in the leg from a turret) to complete this.
Based on in-game information, Johnson is shown to have grown frustrated and jealous of his competitors despite receiving significant funding for the company's technology achievements, and slowly lost his mind over the years until his death from moon dust poisoning.
It is revealed in Aperture Desk Job through retroactive continuity that, shortly before his death, Johnson’s consciousness was transferred to a computer encased in a clay bust of his own head.
Caroline, voiced by Ellen McLain, is Cave Johnson's personal assistant according to the audio recordings heard during the second act of Portal 2.
Greg appears as Cave Prime's assistant in the "Perpetual Testing Initiative" update, but is only heard through grunts.
Within the second game, Wheatley accidentally reawakens GLaDOS, and eventually convinces Chell to initiate a core transfer to replace her with himself.
Aperture Science Sentry Turrets are sleek, thin stationary robots equipped with machine guns that will fire upon the player upon entering their line of sight.
Voiced by Nolan North, these speak with a Brooklyn accent, acting tough but incapable of actually firing bullets or causing damage.
Due for incineration, the turret will give cryptic premonitions of future events in the game if rescued by the player.
After Wheatley seizes control of Aperture Laboratories in Portal 2, he creates the Frankenturret, an amalgamation of weighted cube and turret only capable of walking around aimlessly.
In a Portal 2 trailer, Cave Johnson describes Aperture Science turrets as being sold in a variety of color schemes to the general public as a home defense item.
At one stage during Portal 2's development, the writers had envisioned the long-disused Aperture facility to have been taken over by the cores, creating their own microcosms in the game.
One core was inspired by the character Red from The Shawshank Redemption played by Morgan Freeman, who would have been completely familiar with every detail of the entirety of a ten-by-ten-foot room, and utterly fascinated by anything outside its sphere of influence.
Another core would have been named Quint after the character in Jaws, and considered itself an "AI Hunter" with GLaDOS as his ultimate prey.
They are equipped with their own portal guns which bears the colour of its user (blue and purple for ATLAS, yellow and red for P-body).
The saboteur is eventually revealed to be a bird pecking at the console's keyboard, which sends GLaDOS into a panic when she recognizes it as the one who tried to eat her during her time as a potato.
ATLAS and P-body manage to shoo away the bird, earning a rare compliment from GLaDOS before she notices eggs in its nest.
In the Portal 2 singleplayer campaign, Wheatley finds both ATLAS and P-body in storage and decided to kill both Chell and GLaDOS and use them instead.
They are differentiated from regular Storage Cubes with small pink hearts on its external surface, instead of an Aperture Science logo.
A Companion Cube with the old design, supposedly the one from Portal, appears at the very end of the game, as it is ejected, charred and singed, from the shed (possibly at ground level) shortly after Chell's departure.
Since its debut, the Companion Cube has become an internet meme, leading to a large series of merchandise and appearances in various other media.