Girl isn't used to physical contact like hugs. Sure Natalie hugs Girl at times and she more or less allows it (or not.) but otherwise... Girl isn't really anti-hug. In fact, she would like to give hugs. To her friends. But she has to force herself.

Strangers? Nah, you gotta know her first. Besides, as a scientist generally speaking being "embraced" either involves a vicious monsters, a hostage situation, being tied up, or that foam stuff that hardens really quickly.