tldr; hyunsu means so much to eunhyuk that he, a neohuman who's supposed to be emotionless, was able to smile at the sight of him before he relearned how to feel in the finale.
full analysis under the cut
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even though it's barely noticeable, neohuman eunhyuk smiled when he saw hyunsu for the first time in almost a year, despite claiming that neohumans have "evolved past the need for emotion"
i'm sure a lot of people are familiar with muscle memory, but aside from remembering certain motor skills, our bodies can also store emotions. this is why we're able to feel butterflies in our stomach when we're nervous, our chest feels heavy when we're sad, etc. it's almost impossible for a "human" being to be 100% logical and unfeeling because the framework by our reasoning is defined by our emotions. this is why eunhyuk could regain all his emotions in the finale.
it's most likely that emotions are pushed deep into a neohuman's unconscious mind the same way traumatic memories are, they serve no purpose to the neohuman. however, both can resurface to the conscious mind with the right triggers. in this case, it's hyunsu.
having been reborn with emotions that have no major importance in his wiring, it's only natural that his body isn't used to expressing any of them. so when he reunites with hyunsu, the happiness that arises in him must be so intense, that his muscle memory makes him subtly smile. these emotions are what's left of human eunhyuk, the one who went back to green home because of hyunsu before he let himself become part of its collapse.
not only that, he smiles A SECOND TIME when he asks hyunsu for help, this time being more obvious.
in contrast to the poker face he had when speaking, eunhyuk really meant it when he said he missed hyunsu, even if he was just talking to his monster.
the scene where he tries to relearn how to smile/properly feel emotions by imitating he and eunyu's family picture just further proves my point. when he tries to make himself smile, it looks unnatural and uncanny. (he's trying his best tho and that's what counts)
additionally, you can see the smile reach his eyes when he was with hyunsu.
going back to the picture, him keeping it also proves my point of neohumans' emotions just being buried in their unconscious mind. there's no logical reason for him to hold onto it. he didn't lose his memories, therefore he didn't forget eunyu. the only reason he has it is because of emotional attachment, and he doesn't realize it. he doesn't realize he's more emotional than he thinks he is in general.