James caught Mime Jr. by accident in Sweet Baby James after it purposefully initiated the capture mechanism on one of his empty Poké Balls that had fallen on the floor. Mime Jr. previously resided with the old couple who took care of an old vacation home of James's family; James went there when his Chimecho fell ill. Mime Jr. took an instant liking to James, keeping him company at night while he sat over Chimecho. James was thankful for this, and he told Mime Jr. that it was kindhearted. When it was time for James to leave, Chimecho was still ill, so he decided to leave it in care of the old couple. When he reached in his pocket to get Chimecho's Poké Ball, the empty ones fell out, and Mime Jr. rushed in. James asked why Mime Jr. would want to go with him, and the couple told him that Mime Jr. likes him and thinks he is a kind person. They also said "Looks like you've found yourself a good buddy in Mime Jr."
In Spontaneous Combusken, Mime Jr. was used by Jessie, under the alias of Jessibelle, in the Chrysanthemum Pokémon Contest. They got through the round by preforming a Hula-Dance that was praised by the judges for showing of Mime Jr.'s cuteness. In the battle round, Mime Jr.'s first opponent was May's Combusken. Though it made use of its unusual Mimic and Tickle attacks, Mime Jr. was ultimately defeated when Combusken was energized by its mimicked Fire Spin and hit it with Sky Uppercut.
In Dawn's Early Night!, Jessilina used Mime Jr. and Cacnea for the Double Performance required during the HearthomeContest. Their performance started with Cacnea using Needle Arm, which Mime Jr. mimicked before using Tickle on Jessilina. Both Mime Jr. and Cacnea hugged and tickled Jessilina, spinning her around the stage. After that, Cacnea fired Pin Missile upwards, with Mime Jr. once again mimicking. As the arrow-shaped pins collided, it resulted in a flashy fireworks display. Their performance got Jessilina a high score, placing her among the Coordinators advancing to the second round.
In Old Rivals, New Tricks!, Mime Jr. was Jessilina's choice for the Performance Stage of the Sandalstraw Contest. When she commanded Mime Jr. to use Teeter Dance, the Mime Pokémon waited before fully executing the move, a strategy which Jessilina adapted from Ursula, who always makes charging poses together with her Pokémon to leave an impression on the minds of those watching her performances. The act of storing up energy before unleashing the move amplified Teeter Dance's effect and got everyone in the Contest Hall dancing. The performance impressed the Contest Judges enough for them to grant Jessilina a spot in the Battle Stage.
Following the Team Rocket trio's journey in Sinnoh, James was forced to leave Mime Jr. at Team Rocket's headquarters as it is not a Pokémon native to the Unovaregion, and would seem suspicious.
Personality and characteristics
Mime Jr., as a Mime Pokémon, imitates what people do. Like Chimecho before it, it regularly inserts itself at the end of the Team Rocket motto saying "Mime, Mime", after Wobbuffet says his name. It is seen fairly commonly out of its Poké Ball, similar to Chimecho, participating in Team Rocket's schemes. Also similarly to Chimecho, Mime Jr. is babied by James and he rarely lets it battle.