Let’s get the big, blue, furry monster in the room out of the way first: Boo from Monster’s Inc. is a cartoon. So there is no “grown up” version of Boo -- at least not until Monster’s Inc. 4 inevitably comes out in 2020, and Sulley and Mike Wazowski return to help Boo’s daughter.
(Make it happen, Pixar! There’s a reason Monster’s University wasn’t as good as the original, there was no Boo!)
But there was a real little girl who voiced Boo and her name is Mary Gibbs.