Happy Book Birthday to Demonosity by Amanda Ashby! This sounds like such a fun read.
The Black Rose--a powerful ancient force--has been let loose and has taken up residence in Celeste Gibson, popular girl at Cassidy Carter-Lewis' high school. Thomas Delacroix is the spirit of a fourteenth-century knight who is devoted to protecting the Black Rose, but he needs a contemporary living being to take on the challenge. That's where Cassidy comes in. She's a quirky high school junior who just wants to dress in her vintage clothes, hang out with her best friend, and take care of her father, who is recovering from surgery. She's the last person who would ever volunteer for such a task, but no one actually asked her. Now, like it or not, she finds herself training before dawn and battling demons at parties, the mall, and even at school. But hey, no one ever said high school was going to be easy.