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Many comments said that this manga is ruined because of the adults, me on the other hands, think that showing those bad adults deepens Cain's and Diana's personality and character development. It MEANT to show how bad the adults are in that era because they don't know better. Not because they're actually bad, and Cain is there to "fix" what the parents can't and won't, which shows how he cared for what the adults ignores and tried to "raise" those kids (not just his sister) into a better person.
At first he was viewed mostly as a siscon who won't care about anything else, but it was proven to be false. Of course he might still prioritize his sister's future, but He is also shown as an actually caring and considerate person to everyone that deserved it. He is fair and gentlemanly in general as long as they're not harming his sister.
Also the whole point of why he's going to school so far from home is because his parents felt the need to separate the siblings as Cain's obsession is seen to be quite severe. This is like one of the rare moments the parents made a strict and correct decision for the siblings. Them spliting for a few years also allowed Diana to matures and learn how to become stronger and survive without his brother.
Then they are back together and met new trouble to show their growth in handling things while continuing the "original" plot.