I have one!
His dad and I are both VERY extrovert.
I have always thought he was an introvert until he stated showing (or rather I started noticing) that he displayed signs of physical discomfort as well such as asking to cut his clothes tags, disliking his socks, tugging at his diapers (especially after a sweaty play session). He disliked swaddling as a baby and was most dificult to care for in the initial stages. Thinking back, it must have been his sensitivity to his experience as a newborn.
Now, before we go anywhere crowded, I will tell him beforehand, explaining the place, people, loud music, etc. I also have to tell adults to refrain from carrying or hugging him, which may come across as rather rude in the Chinese culture but what the heck....better than a yelling toddler.
Usually, he would just tree-hug my legs until he feels better
As long as strangers don't approach him, he is pretty much alright. He scored a 13 on the Aron questionairre as well. Others criterias/questions were not applicable to his age (23 months). So I will just have to wait and see.