1) Apathetic 2) Impassive 3) Reserved 4) Restrained 
[adj] - not given to open expression of emotion
undemonstrative, undemonstratively 1. Not open to exhibition or expression of emotion; especially, of affection. 2. Restrained or a tendency not to show emotions openly. 3. Not disposed to expressions of one's feelings; reserved. This word group is directly related to the monstro-, monstr- unit.
adjective not given to open expression of emotion
not given to open exhibition or expression of emotion, esp. of affection.