The Rat appeared only in the special episode Mole in the City. A "Rat", like a mole is another name for an undercover agent. He wears a fedora and a trench coat. He was The Mole's arch enemy.
While not much is known about the story, it is assumed that the Rat is the villain while The Mole is a heroic character.
He wanted to steal The Mole's suitcase without even knowing what was inside. He took advantage of The Mole's blindness and stole his briefcase (though due to the plot being vague, it is unknown why he did so). His eyes are different to those of normal characters, as the pupils are completely round (most characters have a quarter missing to make them cuter) and he has red irises.
He died when the giant gerbil ball he was riding in crashed into a mouse trap factory. He is covered in mouse traps and he is trapped under a giant mouse trap. He sees the briefcase lying on the ground near him. He crawls over it, stretching his spine and other organs out as he goes. When he opens it he sees, much to his horror, that there are nine flashing Mole Bombs (they flash when they have been detonated). As he is trapped in the giant mouse trap, he dies when the factory is blown up.
His death might be permanent, as no other spy episodes of The Mole have been released. If there are any The Mole spy episodes in the future, it is possible he will return.
Mole in the City: It is unknown if The Rat actually killed anyone or not. When he rides off in his giant hamster ball, he runs over a car. He might have killed the occupant of the car, but the car was parked so there is a chance that no one was in it. Whether anyone was in the car when it was crushed. (Debatable)