Like, they kept some of their personality to themself and someone close to them. Not showing it all to the fans
For me, even though Mina and Tzuyu is the most silent members, they are not unknown. In Radio Star they stated that Mina's voice is small and Tzuyu just don't talk much
The most unknown member maybe Dahyun and Nayeon. For Dahyun, they said she doesn't talk much when they are in their dorm, a complete 180 deg when she is in variety shows. As for Nayeon, there must be something going on in her head when she is silence by the look of her eyes (now I know why Tzuyu wanted to know what's inside Nayeon's head 
). She is also brave/assertive? She have had scolded Once/people that always calling (phone) the members
I think idols doesn't have to show everything to their fans. I'll be glad if I know that I don't know their real faces
