The website notes that the scientific term for this is Sensory-Processing Sensitivity. A pubmed search brought up 8 papers using this term. This paper. quoted below, in particular may be helpful as it discusses some of the personality markers and how they can manifest themselves in a person. For instance shyness, stress (especially in situations with lots of people), and possibly feelings of poorer health. I should note also that SPS and introversion seem to be closely tied, but SPS seems to be may not be exactly the same thing.
The Highly Sensitive Person (HSP) scale is a measure of sensory-processing sensitivity, which is conceptualized as involving both high levels of sensitivity to subtle stimuli and being easily overaroused by external stimuli. The current study examines the relationship between an individual's sensory-processing sensitivity, self-perceived stress, and phsyical symptom reports. Results indicated that sensory-processing sensitivity is positively correlated with levels of stress and symptoms of ill-health. After controlling for self-perceived stress and gender, the HSP scale added significantly to a hierarchical regression model predicting self-reported health.
Source: The Highly Sensitive Person: Stress and phsyical symptom reports, Grant Benham, Personality and Individual Differences 40 (2006), p1433-1440/
What may be important to remember here also is that this is ONE aspect of a person's multifaceted personality that can change from day to day, in certain situations with certain people and probably over time as well.