When they're not harassing Kim, they're making trouble with their next wacky science invention, launching rockets, or sending out screensavers of an unexpecting Kim to all of Middleton.
The twins were born sometime during Kim's pre-school year, although Kim though the boys would be a single girl. At the age of twelve, they were admitted to Middleton High School a couple of years early.
Jim and Tim have not, to date, appeared apart. Their lines and actions are mostly scripted as if for a single character, and they exhibit a somewhat creepy level of synchronization. Antagonistic, anarchic, and brilliant, the tweebs care little for rules and restrictions on their activities, and rebel against their older sister's attempts to rein them in. They use a set of invented slang between themselves, which is their trademark sayings: "Hicka-bicka-boo!" and "Hoo-sha!", used as a call and response interchangeably.
- Child Prodigy: Jim and Tim are proven to be gifted children with a high level of IQ which they skip a grade level to Middleton High School ahead.
- Genius-level Intellect: Their intelligence was high enough that they consistently proved smarter than children their age, students above their age, and even adults far beyond their age.
- Mechanical & Science Engineering: Jim and Tim are mechanical and scientific prodigies, as well as gifted problem solvers. Even at the tender age of ten, they constructed an array of amazing and often destructive devices out of the parts made available by their father's experiments, or "borrowing" their father's work materials. While many of these inventions exploded, ran wild, or were halted in progress by others, a number of them reached completion and proved far more effective than those designed by older, more experienced scientists. The twins displayed a preference for regularly practicing in their father's field of rocket science, but also had considerable talent in automobile repair, despite no known formal training in either area.
- The boys' full names were never given in the entirety of the series, and while it makes sense they are named after their father James Timothy Possible, it is not supported by canon as to what their full names were, and just as possible "Jim" and "Tim" are not short for anything.
- Their voice was originally provided by Shaun Fleming. Since their redesign, it has been provided by Spencer Fox, best known for voicing Dash Parr in The Incredibles.
- In the German version they are called "Zwombies", a portmanteau of "Zwillinge" (twins) and Zombies.
- In the fourth season, there are some noticible changes. Jim and Tim no longer dress alike and are drawn older.