I thought he did that because he was upset he didn't have a mark. Moreover, I think in some chapter it was mentioned her mark did not resemble those other soulmate marks. And she healed aseph with some weird power from her mark. So I think while it is obviously aseph related there's more to this mark.
same! i left a comment about it in past chapters too: i think since she wasn’t supposed to be in this novel universe (“her character” didn’t exist in the original novel) aseph’s mark isn’t supposed to exist too and that’s why it’s hidden?
i also believe it will appear somewhere in the future due to her presence and strong affection/power being “validated” in the novel universe (most something likely related to the goddess?)
Wait wasn’t it established that she was originally from the novel universe and in the novel and the reason we don’t know of her is because each time she dies exchanging her life for his which results in him having an unrequited love for his sister but because even when she was placed in a different universe she was so entangled with him that the goddess couldn’t intervene in the slightest besides keeping her alive this time so she can finally achieve love and happiness
I think it will all nicely tie up with the reasoning behind her seal because most plot driven stories do do that. But right now the main point of focus seems to be the crown prince's arc, aseph's ability to use mana despite having 'black magic' affinity, and the weird dude out there stealing seals. Also the story never addressed how the seals were taken, that is, if both holders of the same seal were hurt or if it was just one of them. And if it was just one of them what happened to the other person who still has their seal.
I noticed in a couple of chapter he was touching his chest in that area where zays is, and also there was the clue from alcester that says her marks is different in some way. I'm pretty sure nobody else has a mark with healing powers... anyways I just have a feeling based on what the goddess said before that her fate was always tied with asephs