Jessie caught Yanmega as a Yanma in The Thief That Keeps On Thieving! after Tyler battled and weakened her. After she captured her, Yanma was sent to Giovanni as their second successful theft. However, he immediately returned her after analyzing her and realizing she wasn't good enough, along with the fact that they already had plenty. Jessie then decided to train Yanma herself—unbeknown to the fact that Yanma already knew Ancient Power, Jessie sent her into battle, who ended up quickly evolving into Yanmega.
She was used often by Jessie, sometimes replacing Seviper as her main fighter. So far she has shown herself to be a strong battler, as she destroyed Pikachu's Thunderbolt with her Sonic Boom attack as seen in A Lean Mean Team Rocket Machine!.
Yanmega made her Contest debut in Battling the Generation Gap! where she was used in the Performance Stage of the Celestic Contest. However, she failed to get Jessie to the next round.
In A Shield with a Twist, Jessilina entered Yanmega in the Majolica Town Pokémon Contest and won her second Ribbon. She was also used in the Chocovine Contest during Another One Gabites the Dust! where she passed the first round with a dazzling performance, while revealing Yanmega's Ability to be Speed Boost.
In Steeling Peace of Mind!, Yanmega was sent out alongside Team Rocket and Barry's Pokémon to confront Team Galactic, only for her to get hit by Golbat's Sludge Bomb.
In Pursuing a Lofty Goal!, James borrowed Yanmega in order to enter the PokéRinger competition which was being held in Squallville. Jessie was reluctant as James failed to win the competition with her Dustox before. Like before, James failed to win with Yanmega.
In Playing the Performance Encore, Jessie used Yanmega for the Daybreak Town Contest. It battled alongside Seviper against Ursula and her Plusle and Minun. Seviper and Yanmega were able to combine their attacks well, creating a Haze-Sonic Boom and Poison-Silver Wind. However, Plusle and Minun were able to defeat them using their Spark and Helping Hand attacks, knocking Jessie out of the competition.
In Dawn of a Royal Day! she was used in the Performance Stage of the Arrowroot Contest where she created an amazing performance with her Ancient Power, placing Jessie among the Coordinators advancing to the Contest Battles.
She was also used in the Sinnoh Grand Festival where she helped Jessie get through to the semi-finals.
Following the Team Rocket trio's journey in Sinnoh, Jessie was forced to leave Yanmega at Team Rocket's headquarters as she is not a Pokémon native in the Unova region and would seem suspicious.
Though not much is known about Yanmega's personality to distinguish her from others of her species, she is shown to have a habit of landing on the heads of people and other Pokémon. She has been shown to land on Jessie, James, and Seviper's heads, and even Ash, Dawn, and Brock's as a Yanma before she was caught by Jessie. Yanmega is shown to be exceptionally powerful compared to the rest of Team Rocket's Pokémon, being the closest of any of them to send Ash and his friends blasting off. In Playing The Leveling Field!, Yanmega is shown to care about Jessie's approval, crying when she couldn't imitate Fantina's Drifblim for Jessie. She is very loyal to her Trainer.