ambivert

ambivertti A person who is neither clearly extroverted nor introverted, but has characteristics of each.
1969, Vladimir Nabokov, Ada or Ardor, Penguin 2011, p. 126:

‘We are not in the same class, in more ways than one’ (laughing); ‘she's a little genius, I'm a plain American ambivert, but we are enrolled in the same Advanced French group ....’