Desi or
Mona seem like nick names her friends and parents would call her by. As someone who wants to be an evil cult member, she might try to sound more imposing by insisting on being called Demona or by her full name only. Since she is still a teenager, you could create an interesting divide between her identity as teen with parents and school and her attempts to become evil as part of a cult. At home she could
Desi or DeeDee and as Demona with the Disciples of Death.