"I made a promise to you and mom that I will never leave your side unlike what dad did. You will not die alone tonight. You will not die at all. Not on my watch." ― Donny to Aurora in Ultima Petitio
Adonis "Donny" Castillo is the male main protagonist of Ephemerality. Donny was born into the Castillo Coven; however, unlike his elder twin sister, he was not the Heir Supreme. He is the son of Rhea Castillo and Erik Santana, and the young twin brother of Aurora Castillo.

At the beginning of the book, Donny was described as a very artistic soul. He is a freelance artist in his free time, the leading activist in Thornwood University, and the band leader of The Nines; a band which he formed with his best friend, Stefan Deveraux. He also works at the Thornwood Bean as a Barista. He was also described as Aurora's polar opposite.

Donny is best friends with Stefan Deveraux, the brother of his former girlfriend, Selene Deveraux. He is also close friends with his sisters' friends, Phoebe McCartney and Mateo Ortega. However, it is noted that Donny despises both Sebastian Young, his sister's boyfriend, and Veronica Mizuno.