Yui caught Wigglytuf, as an Igglybuff during her travels with Ritsu, Mio, Tsumugi and Azusa.
It was revealed in a flashback that the Light Music Club was doing a live performance concert, until a playful Igglybuff hopped over to the stage and began dancing, After the concert, she decided to join Yui after calling it cute which made Igglybuff blush, and so she caught her in a Poke Ball.
She evolved into Jigglypuff in another flashback after Yui was in danger. As a Jigglypuff, She shares the same traits as Yui, She is charming, she also gets pouty if she doesn't get what she wants like Yui does, she is often childish most of the time, but she's focused on performances, and really affectionate to Yui.
After some time, she used the Moon Stone and evolved into Wigglytuff in Jumping Over The Moon Stone!, she is a concerned type and worries about Yui if she has some trouble with Geeta. But she helps Yui in any way to avoid the burden. In "What's In a Nickname?", she was officially nicknamed Wiggy by Yui, and even nicknamed the other Pokemon she has.
Wiggy's known moves are: Hyper Voice, Dazzling Gleam, Sing and Play Rough, and her ability is Cute Charm.