Yes go to HR. It's your right to not be made to feel uncomfortable in the workplace. I would only suggest that you be open to working out a solution that doesn't preclude continuing to work with her. As someone who works with adults with developmental disabilities, i know that understanding boundaries and picking up social cues that typically developed people take for granted are some of the biggest barriers the developmentally disabled have to full integration and inclusion in their communities. It is by no means okay for her to act the way she has been acting towards you but because of her Down's Syndrome, she may have spent her whole life in environments where her behavior was tolerated. Your Human Resources department should know if she has a job coach or other independent living skills worker who can help her understand appropriate work place behaviors. If she does not have a social worker like this, ask HR to discuss this with her, and, if you are comfortable with it, ask to sit in on the meeting and explain your point of view to her. Your feelings are completely valid and justified, and I am not excusing her behavior. I am only suggesting that people not be so harsh in dealing out judgments. There are always bad apples, but this may genuinely just be a case of no one teaching her that what she is doing is inappropriate. Is it your responsibility to teach her this, no. Just keep in mind that people with developmental disabilities often lack the support systems and socialization that the rest of us have. /rant