The thing that I realised first from this series is the main heroine herself. It resembles Karin--appearance only--in one way, a lot. Well, I think it's kind of the norm that if the girl is the strong-cold type, the main protagonist is ought to be the naive, crybaby one.
Nevertheless, I sometimes get irritated when he keeps on that "can't everyone be friends?" and "Seine-chan!" attitude, but I tolerate it because he's strong. In another way.
For the plot, I really really like it! It is very creative to use the "Seven Princess of the Seven Seas' (reminds me of Mermaid Melody tho) combines with the "Little Mermaid" story, in which the youngest mermaid lost her voice to the witch. Just like how Seine took Aion from the mermaid.
I also like the extra chapter where the story is kind of related to Chibi Vampire. I also like Chibi Vampire and still complaints about its bittersweet ending, but nah, that is for another place, another time.
The one thing that feels missing from this manga is the part of their last battle. It feels rushed and illogical when Ariel is attached to Seine just because she ate Shion's heart. Okay, that explains some part, but it just feels.... weird to me.
Overall, I really like this mangaka and Hekikai no Aion itself <3 Again, the mangaka managed to make such a bittersweet ending. I like it, but at the same time, I loathe it. But I guess, this kind of ending is the most fitting to close the curtain of the manga. Salute.