I think the author is doing that on purpose, I think he's supposed to be mostly devoid of emotion. Before, his only motivation to get stronger was for his mom and sister so now that his mom's healthy I think he's gonna either find a different motivation or do it just because.
Remember, early chapter where he went to a C level gate together with his Vice-press, when he killed the 8 hunters he said he felt that his emotion is slowly dying or something, also the chapter where he killed the A class hunter of the association, he also said that one of his emotion died again. In my opinion, the reason why he looks cold is because of his powers. About hunter Cha, I think it curiosity turns into love because he saved her many times. The romance in this manga is actually not big, and won't even affect mich of the story plot.
I feel like the girl is going to help bring back his emotions which are a side effect of gaining so much power and killing so many people
He has an excellent poker face. If you read the novel you'll notice that he feels strongly about a lot of things but he's just good at hiding it.
I have just realized that we dont really know much about the ML. He does not seem to have any real emotion, does not cry, laugh, angry, sad. His face is almost always cold, uninterested, plain, edgy, emotionless. At first, I think that was kinda dope and cool but not knowing what his goal, motivation, hobby are, I feel this is just a machine with superpower walking around. At least, before he became strong, he still seemed human and has emotion to me, but now.... Btw, The girls that fall for him probably just impressed by his power, not his personality, cos neither the audience nor them know what it is. Like this story really has the potential to be much greater, deeper character development!! （/TДT)/