When someone adapts their personality, behaviour and taste to fit into the role of someone else, commonly used to describe someone immitating a Twilight cast member.
"Have you seen Ashley lately? She is such a monnor-cacneil. She even ordered the exact same prom dress as Bella has off the internet."
"Adam is being such a monnor-cacneil. He even looked into sharpening his incisors the other day. So weird."