Adele

Arceus aka Adele, however, most know her as Arceus. Though stated in myths and legends, she did not create the world. Luca asked who she was and she replied "I’m just His servant", which means she isn't an idol.

She has been seen by Ash, Dawn, Brock, Sheena, Luca, Lucian, Cynthia, Stan, Émile and Flint; however, she can be summoned by Mitternacht freely.

She usually appears in the form of a dream, rarely seen, so she eludes capture. She gives people the ability to understand Pokémon and natural disasters such as earthquakes. She has limits, so she accidentally switched Lucian and Cynthia's bodies and couldn't fix it because of this reason.

She can also change the tone of her voice, depending on whether she is happy or angry. Though she can speak via telepathy, she can speak normally like any other Pokémon can.

Pegasus
In circa 200 B.C.E., Adele once had a brother named Pegasus but he was killed by meteorite that was half the size of a house. Adele was trying to help him destroy it. She was successful, but he wasn't. He bled to death but she couldn't save him because God limited her powers. However, God spared him and sent him to be a guardian of space.

She hasn't seen him since that happened, but she knows he's still out there in space, somewhere. She isn't sure where. Her brother was a shiny Arceus.

Form changes
Even though she is the last of her species, she can change her form depending on what plate is attached to her: