WordNet sense 1 (showing little emotion):

phlegmatic

WordNet sense 2 (having or revealing little emotion or sensibility; not easily aroused or excited):

impassive

WordNet sense 3 (lacking warmth or emotional involvement):

dry

WordNet sense 4 (cool and formal in manner):

reserved, restrained, reticent

WordNet sense 5 (not given to open expression of emotion):

undemonstrative