I see Umibe no Cain as a story about the reciprocity of love, just like Cain and Abel. In the Bible the story goes: Cain and Abel brought offerings to God, but God only respected Abel's offering while Cain's was 'not respected'. Cain got mad and killed Abel in the field, lying to God when asked about it. God then cursed him to wander the earth and whoever kills him will be avenged sevenfold. God's respect is a metaphor for the world itself. When you offer something out of genuine love to this world, you wouldn't expect anything in return. The point is to do something goodcompletely for the sake of the other even if it goes against your own well-being, because the desire to avoid self-sacrifice is already an expectation of a return. This is the divine love (agape) that Christianity is centered around: the idea of an infinite, undeserved grace or gift unbounded by the transactional nature of mortality. By giving without a return, it means that it's free and unlimited.

Cain's expectation of reciprocity distorts his idea of love to become limited, transforming God's 'non-respect' into disrespect, when it's actually just the nature of the world. Good and bad things happen, and we can't see the bigger picture or the chain of cause and effect that led up to it. However, we can humble ourselves before this bigger picture and decide not to fixate on its rationality--to transcend cause and effect by refusing the requirement of a favourable effect and surrendering yourself to God's 'plan', God's 'respect', or the world in its totality (if you don't believe in God).

So the message is clear, to love is to not expect anything in return and to show understanding or acceptance to the state of the world which is inexplicable to a single human. To deny this is to live in suffering, to bear the mark of Cain and inherit the sin of murder until the end of time.

Umibe of Cain's characters oscillate between the roles of Cain, Abel, and God. However, in this case, God is not divine, but an imperfect human who holds power.

Both of the MCs, Nobuko and Sano, take all three roles simultaneously depending on the perspective. In Nobuko's eyes, she becomes Abel and Sano becomes God: she is respected by the one she seeks love from, as Sano listens to her problems and shows her care. In Sano's eyes, the roles are flipped, with Nobuko being the one that gives her love, complimenting her dresses and coming to her exhibitions. In terms of personal freedom, Sano becomes Cain and Nobuko becomes Abel, as Sano envies Nobuko's carefree youth, singing part-time in a cafe, loitering around town. In terms of stability, of course, the roles are flipped again.

The most important assignment of roles is based on gender roles, sexuality, and society in general. In relation to her family, Nobuko takes the place of Cain, her sister Abel, and her mother God. She is disrespected by her mother, the one from whom she seeks love, for her lack of femininity and normalcy, while her sister is respected for being her opposite. Similarly, in the eyes of society, Sano is treated favourably due to her femininity, while Nobuko is viewed as an oddity. The climax of the story occurs as Nobuko realizes her sexuality--will she be accepted or not?

Well you have to read it to find out. Either way, the conflict lies in the intent of their actions and the reality of life. Did they actually love each other? Was the love really unconditional? Is complete unconditional love in this life even possible? Do we have to live with our imperfections? Is there no choice but to suffer and move on? The answer depends on what you believe.

Now, whether or not the manga communicates this well depends on you (or maybe there's a different message). I think it did a pretty good job. I just wish the visuals were more striking like other old shoujo considering the heavy subject matter. There should be more panels where Nobuko curls up inside a dark, growing bramble of thorns or something lol.

Overall a good read. 6-7/10.

note:
- idk why this turned into a weird sermon, pls forgive any theological mistakes I made