Let's see, Loner is a person who avoids the company of others and/or prefers to be alone.
Theoretically, a character doesn't have to be Kuudere, Dandere, Shy or Impersonal to be Loner. I'd say that the Loner doesn't need communication with other people as much as others, but it doesn't mean that s/he is shy or introvert.
However, 1) we don't have stat responsible solely for communication; 2) if we going to consider communication part separately, we need an opposite trait as well, and we already have one: Charismatic, with Shy as opposite. We have 84 traits already (yay!

), I'd prefer to add only well-established ones.
And 3) it could be implemented good enough via effects, but I have no idea how. We don't have social needs like in Sims games, so I don't see any good options.
If there won't be any objections, I will remove it within a day.