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.
Diamond & Pearl series
In Dawn's Early Night!, Jessie used Cacnea and Mime Jr. in the appeal round of the HearthomeContest. Cacnea used Needle Arm, which Mime Jr. Mimics before using Tickle on Jessie as both Cacnea and Mime Jr. hug and tickle Jessie, spinning her around the stage. Cacnea then fires Pin Missile, which Mime Jr. mimics, into the sky and creating fireworks. This performance gets Jessie a high score, advancing her to the next round.
In Old Rivals, New Tricks!, Jessie used it during the appeal rounds of a Contest. Jessie commanded Mime Jr. to use Teeter Dance, and after storing up energy for several seconds, it let out a massive version of the attack that got everyone (the judges, audience and even Marian) dancing. The impressive display allowed Jessie to progress to the finals.
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 Unova region, 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". 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.