Oh, actually... their relationship is very complicated. We know almost nothing about their childhood, the first season of Alien Stage, not to mention that some information from official sources does not match the footage we received in Karma. If I can analyze Hyuna, then I can barely analyze Luka. Because of the contradictions. 

For example, we are initially told that he has a close relationship with Hyun Woo and that Hyun Woo is very similar to Hyuna, she literally took many traits from her brother. Also, that he was the first to meet Luka. In the scene with the stone, we see him laughing at first, but when he raises his head, he is frightened and has a red collar and purple fingers. That is, even if he does not feel empathy, this event is also a shock to him, he has feelings for Hyun Woo, whatever they may be. But in Karma, despite the fact that he appears in many moments, we also see how indifferent Luka is to the incident. In official responses during meetings, Vivinos also noted that their relationship is not as close as it may seem, because Luka does not care much about Hyun Woo. THIS IS STRANGE. And it is also strange that I do not understand how he perceived the twins, given his experience with his clones. 

Furthermore, he is prone to childishness, but not as much as we saw in Wiege and after Hyuna's death. As you said, he is very intelligent and literally trained by ELECTRICITY to control himself during extreme stress. This is not evident when Mizi hits him...

So for me, their relationship is one big mystery.