Gardevoir, as a Ralts is Mio's first Pokemon she caught thanks to Teresa's help and debuted in the episode Lunatone, Mitsuki's All-New Friend!, she is confirmed female.
She appeared alongsde Mio, she used Confusion to lift multiple heavy luggage despite her size, but the Confusion was so strong it transformed into Psychic, thus learning a new move in the process.
As a Ralts, she is very timid like Mio and gets easily embarrassed like her Trainer, but she cares for Mio as lot. After some time, she evolved into Kirlia and learned Attract in Dancing With Graceful Fairies. As a Kirlia, she is very graceful and very affectionate and her Psychic attacks are twice as powerful, she is quick to judge and gets embarrassed when Mio gets embarrassed too.
She finally evolved into Gardevoir in Embracing The Fall after Ritsu falls off a cliff and Mio jumped in with her, but Gardevoir saved Ritsu and Mio just in time, thanks to Mio's bravery and selfless acting, the Light Music Club was presented a Key Stone for each girl, allowing Gardevoir to Mega Evolve, and Mio tried just that and comments how beautiful she was in her Mega Gardevoir, Mega Gardevoir slightly got embarrassed of all the praise she was getting.
As a Gardevoir, she is very elegant and will protect Mio at all costs, she is highly affectionate and shares the traits of all the girls in the Light Music Club including Jun and Ui.
In "What's In a Nickname?", she was officially nicknamed Darling by Mio, and even nicknamed the other Pokemon she has.
Gardevoir's known moves are: Hyper Voice, Moonblast, Psychic and Energy Ball, and her ability is Trace. As a Mega Gardevoir, her ability is Pixilate.
She has a Modest nature, and its characteristic is "Alert to sounds."