lol many are getting confused by those chapters since it's not very clear who the mermaid is what's happening.
long story short version:
*I'm really bad with names so try to be understanding
Mermaids are the highest on the hirarchy of maderury, they are considered royalty even. Because their soul is so strong and 'heavy' they can't make a physical form made of flesh (such as a human body) so contain their soul. so they remain forever in thei soul aearance state. that is, unless they are tought how to control their soul apearacne.
There are families of heavy seeds who serve those royals. such as the Kunimase family and that bears family from Canada (that's why they were called to the comming of age ceremony).
Kunimase's older bother, forgot his name bleh, is very good at teaching others to control their soul spearance as we can see in the training at the beginning of the story. though the mermaid thing happens before the plot of sex pistols takes place.
he is sent by his father (Kunimase's mother, remember that blond freaky woman? Maiko or whatever) to France to teach this mermaid prince to control his soul apearance. That's what he is trying to do there inside the weird castle.
clear now?
Yes! That makes a lot more sense! So the mermaids are on the highest control and that's why there's that coming of age thing. So do you know what was happening at the end of their story? When Kunimase's older brother is slashing the mermaid with the sword. Was that to get rid of some sort of evil??!
That sword was made especially for soul apearance state. In human terms it can be defined as a 'ghosts sword'- a sword that can physically harm and bring pain on soul apearance. It doesn't really harm the soul since it's not a physical thing, but the soul still feels the pain.
The guy used this sword to punish, to earn respect or to simply try and make the mermaid back off.
I got the impression that he was feeling a sort of love/hate thing for the mermaid, and was going slightly insane, so he lashed out. Like a murder/suicide or something... I don't know that whole storyline is strange. I plan on rereading it once vol 9 comes out.
Ah sorry, but can someone explain what happened in the mermaid story?? I got really confused ;v;