Dr. Fen

"So what?! I got the reputation, I got the robot! YOU STAY AWAY!!"

- Dr. Fen revealing his true nature

Dr. Fen is the main antagonist of the Kim Possible episode, Grudge Match. He is voiced by Tom Kenny, who is known for voicing SpongeBob SquarePants.

History
Dr. Fen was a co-worker of James Timothy Possible (the father of Kim) in the Middleton Institute of Technology. Despite his fair relationship with Dr. Possible, the latter is confused of how Fen was able to obtain a perfect position since he is an incompetent robotics expert. This was shown as the robots Fen built would only cause harm or damage to himself, much to his frustration.

When news reports that a robot was stolen from the Institute, Kim and Ron Stoppable met Fen, who explained that one of his robots (which was design with self-preservation) was stolen by his former lab assistant Vivian Francis Porter, whom he fired because she 'couldn't keep up', though Dr. Possible claimed that Vivian had quit the lab because she had enough of Fen. Searching around through the lab, Kim find a piece of hair which actually belongs to Vivian.

Kim learns that Vivian is currently a members of the robot fighting club called the Robot Rumble, so she heads over there with Ron and her cousin Larry (who is a member of the club) to confront her. However, Vivian angrily denies that she had stolen the robot from Fen, and is backed up by her boyfriend Oliver, who tells the gang to leave. However, Kim and Ron are able to retrieve the robot from the arena and return it back to Fen, who is delighted.

However, the facts behind the caper start to make Kim suspect of something wrong when Dr. Possible comments on how clueless Fen can be over robotics despite his position. This was backed when her friend Wade searched for any work that Fen has done; as it turns out, Fen has never got any article or scholarship on robotics, and that Vivian has accomplished them instead. Kim then realizes that Fen is the real theft as he wanted to get credit for Vivian's work of making a thinking robot, and that he framed her in order to get his hands of her robot.

The next day, as Fen is deciding to figure out how to activate the robot, Vivian and Oliver arrive to confront him for stealing the robot. When Kim and Ron arrives, Fen asks the two to call security, but Kim reveals that she is aware of the fact that Fen staged the entire theft to take credit for Vivian's work. Realizing that he's been exposed, Fen angrily tells the four to stay away and tries to escape the lab with the robot, but Kim uses a giant electromagnet to sweep the robot from his hands. Oliver ends up being pulled into a magnet as well, revealing to be a robot built by Vivian. However, after Kim deactivates the magnet, Fen grabs a mallet and is about to destroy both Oliver and the robot, but one of his robots grabs him by the lab coat, just as Dr. Possible and the police arrive to the scene where Kim explains the truth to them.

It is implied that Fen is sent to prison on the charges for fraud and theft while Vivian is reinstated back into the Institute to take Fen's place as Dr. Possible's new co-worker.