Doug Rattmann

The Rat Man is a cut character from Portal who has run through the same Test Chambers that Chell has gone through. Absolutely nothing is known about him, except for his obsession with the Weighted Companion Cube. The Rat Man seems to only live off of beans, milk, and water. The bean cans have an Aperture Science logo on them, meaning that it's possible that they and other food supplies were obtained from a cafeteria or similar area within the facility. The Rat Man has left many clues in small spaces that help Chell survive in the Aperture Science laboratories.

Most of what is known about the Rat Man's character is revealed in his den in Test Chamber 17, and is as follows going from left to right starting at the entrance of the den:

"The Vital Apparatus vent will deliver, oh it will WILL. The weighted companion cube DOES speak. Superstition, perceiving inanimate objects as alive, and hallucinations. I'm not hallucinating. You are. The companion cube would never desert me.  Desert.  So long... The companion cube would never lie to me.  NEVER.

Login: cjohnson Password: tier3

Trust me

Where are you? I will find you. (Separate words mixed with hearts and pictures of the companion cube)

You said to take care of it. How can I you wouldn't let me. I should disregard your advice. Leave me alone! STOP WATCHING. Though earth and man are gone, I thought the cube would last forever. I WAS WRONG. (Italicized phrase is a possible reference to the Combine rule over Earth. Excluding the italicised portion, this is a satire of W.H. Auden's "Funeral Blues". )

Because I could not stop for Death, he kindly stopped for me The cube had food and maybe ammo and immortality (Satire of "Because I could not stop for Death" by Emily Dickinson)

Why? (written in red 18 times with several question marks)

Not in cruelty, Not in wrath, The REAPER came today; An ANGEL visited this gray path, And took the cube away" (Satire of "The Reaper and the Flowers" by Henry Wadsworth Longfellow)

(The letters in bold seemed to be underlined specifically for unknown reasons)

The login name and password (which are referring to Cave Johnson and his 3-tier plan) give an explanation as to how the Rat Man opened up certain areas of the Test Chambers, gained access to other areas of the facility, etc.

The scribblings also show that he is possibly an educated man, quoting both Dickinson and Longfellow to express himself over the loss of his weighted companion cube.

Aside from the his scribblings, the Rat Man appears to have adept technical skills. Using the login name and password he managed to procure he was able to open up certain wall sections in Test Chambers, make vital apparatus vents dispense more weighted storage cubes, and take pictures of the companion cube (likely using the cameras in the testing chambers). Other than that, he seems able to create makeshift stoves out of dismantled computers, upon which cooking pots are set on top of.

The Rat Man's fate is unknown. His hints stop just prior to the big sentry ambush near the end of the game. This could mean one of several things: It could mean that he was killed due to the ambush, that he got past the ambush only to be killed by GLaDOS, or that he simply hadn't gone into the area yet and is still roaming the facility.

Behind the scenes
He was originally to be seen in the game but only survived through his scribblings.

Trivia

 * The Ran Man wrote the infamous sentence "The cake is a lie" and the arrows that show Chell the way.
 * In the commentary, Valve hints out that there are other people trapped in the facility.
 * One writing on a wall shows a picture of one of the cameras and above it the words "she's watching us', "she" referring to GLaDOS.