It looks like the LAEM forum is locked, so I'mma post this here.

No one actress should be cast as Rei. Instead, she should be a composite of several people, similar to Darth Vader. For example... any random 14-year-old model for the body, with Ariel Winter's face CGI'ed onto her head, and the voice of Jennifer Tilly or Emma Rigby.

A secondary benefit of this approach is that it would add to her creepy alien-ness.