Mesprit

Mesprit is a legendary Pokémon known as the Being of Emotion and was first introduced in Chapter 17 of Stun. She was created by Dialga and Palkia in the dawn of time along with her brothers, Uxie and Azelf. Mesprit was assigned the role of creating and monitoring the emotions of all Pokémon, which eventually proved to lead to her insanity after about 1700 years.

Prior to descending into madness, Mesprit had found a lone Squirtle named Blu in a Pokémon Daycare and decided to raise him as her own. After a few years of this, however, Mesprit had accepted had something bad was going to happen to her and decided to let Manaphy raise him. Before leaving him with Manaphy, however, Mesprit blessed Blu with eternal "sweet vibes".

Mesprit decided to accompany Stun on Eevee Island after the war against Tenebrae, and actually proved to be a valuable fighter when battling Mew after she had been compromised by Team Rocket towards the end of ''Stun II. ''

Mesprit seems rather confused at all times, making wild accusations such as accusing Mew of stealing her boyfriend and confusing Stun for a Pidgey, Rattata, Weedle, Meowth, Houdour, Goldeen, Igglybuff, Gastly, and Ledyba. She has also been shown threatening to leave Stun completely devoid of all emotion, but has not demonstrated this ability as of yet.

Mesprit was captured by Team Rocket along with Mew during Stun II, but was somehow able to escape before she could be experimented on. It's possible that the Dark Ball had no effect on her, due to her ability to control emotions.

It is revealed that some time between the events in Stun II and Project Stun Mesprit has been captured by humans.