Squeaky is an albino lab mouse, used for testing on the structure of the rodent mind. He has white fur, and pink eyes and skin. Attached to his head via thimble is a strange piece of gadgetry possibly used to transmit rodent thoughts into a computer.
Squeaky's first appearance was in the short "What's Up, Doc?". When WordGirl playfully wiggled her finger in Squeaky's cage, he attempted to bite it, and the super-heroine used the opportunity to define the word "vicious". At the end of the short, Professor Boxleitner patted the top of Squeaky's cage and ominously said "Soon, Squeaky. Soon." It is possible that Boxleitner meant that he would soon be able to read Squeaky's mind.
Squeaky's mind was merged with Professor Boxleitner's very early on in the series, in the WordGirl short "The Crazy World of Doctor Two-Brains", here Boxleitner planned to use Squeaky for an experiment that he hoped would allow people to read the minds of mice. The distracted scientist left the rodent alone for a brief moment to check on his pastrami sandwich, and the vicious lab mouse pressed the button on the dashboard specifically labeled "Holy Cow!! Don't Press this Button!", merging the mouse's brain (as well as some of his DNA) with that of the scientist, creating Dr. Two-Brains.
Squeaky was briefly seen in The Narrator's flashback in "A Game of Cat and Mouse", when the narrator was recapping to the viewers how the good Steven Boxleitner became the evil Dr. Two-Brains.
In the episode "Mouse Brain Take-Over," Dr. Two Brains decided to let Squeaky take over his mind so that he may wreak a little extra havoc. Squeaky turned out to be extremely evil, quick-moving, more than a match for even WordGirl, but at the end of the episode, Dr. Two-Brains re-entered his mind (albeit with no memory of what had happened). The reason was that his henchmen didn't like the change.