Pikachu

Pikachu is a Kakuna named after Red's favorite Pokemon.

It appeared in the first episode of the series where it was used as a paperweight by Prof. Tree. Rusty took it as his starter Pokemon and uses it throughout the next few episodes of the series. It was later robbed by Team Rocket, when it evolved out of pure happiness from them touching it. It then slashes Rusty and goes with Team Rocket. However, Rusty still thinks it loves him and sets off to save him.

When Rusty infiltrates Team Rocket, it steals a gun and attempts to shoot Rusty. The gun then runs out of ammo, but Pikachu still attempts to shoot it. In the next episode, Pikachu is in the back of Team Rocket's truck, still trying to shoot the gun. When the truck explodes, Rusty thinks Pikachu is dead, and begins to mourn his loss. He then realized Red left an autograph and forgets about it.

However, Pikachu appears again, explaining to Rusty (who still doesn't realize Pikachu hates him) that he survived and killed Giovanni, becoming the Viridian City Gym Leader and the boss of Team Rocket. However, the police arrive and hold Pikachu at gunpoint, explaining to Rusty that they have been tracking "the crooked Beedrill" for months. However, they explain they're still looking for his master because they can't legally prosecute a Pokemon. Rusty then says that he is Pikachu's master, and the police arrest him.

Trivia

 * There are several hints at Pikachu surviving the fire before it is actually shown. In both Battlehaus and How To Breed The Perfect Pokemon, it is mentioned that Team Rocket has a new boss. In addition, Pikachu didn't appear in the Ghost Tower with Rusty's other dead Pokemon.
 * Although it is usually Rusty who is blamed for Pikachu becoming evil, the fact that Prof. Tree used him as a paperweight may have also been a factor. But it should be noticed that Rusty's treatment was far worse since he threw random stones at it to find an "evolution stone" for Kakuna, collared and dragged it all the time or made it use "Headbutt" by throwing it against a trainer's Rattata.
 * As of Viridian City Gym, Rusty still believes Pikachu cares for him.