I need some help with the name of my character....?

Okay so my characters name is Sarah Madelyn Belle. She comes from a family of very upitty people and I wanted her name to be somthing more unique. But I've foudn that I can't rename her, she is already Sarah in my head. So I wanted to give her a longer name with Sarah in it but I don't wnat the name to be hyphenated. I came across seraphina but I don't want to type Sarah as "sera"/. is it okay to have her full name be Seraphina but have her go by Sarah (spelled that way)? Is there any other way of spelling Seraphina?