Serial killer in my thriller story?

I'm writing this fic and I decided to add a serial killer into my story as it is a thriller.

However, I'm not sure who the character should be exactly. :/

Should it be a new character? Someone already in the story?

And how can I pull it off? I mean, how should I write this person?

I was thinking of making the killer a relation to the main character, but I think it's too much... >.<

Any suggestions?

Thank you. ^_^