This に is same as this "に"　" 脇役になる”　: going to be supporting role. It means "光のスカート is going to be upstaged by the girl".

Hi. I’m Japanese high school student. “脇役” means a person doesn’t play a main role in the story. It’s used when you want to describe something don’t stand out. “霞”is a kind of whether. Maybe “misty” or “ hazy”. When it’s hazy, you can’t see anything, right? So “脇役に霞ませる” means to make something out of your attention in this text. I guess This text is from a novel. Novels have lots of unique expressions. Good luck