These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The Woobie: Oh my God, so many in just four volumes. Big Eyes from Dream's End, Nae from Mermaid's Promise, Misa and Yukie from Mermaid's Scar, Natsumi from The Bone Princess, Akiko Kiryu, Sugiko, and the old woman from Mermaid's Gaze, and the older Nanao from Mermaid's Mask.
Jerkass Woobie: Towa Kannagi. Despite all her pointed cruelty towards her sister and willingness to kill for her goals, you have to realize she figured out many years ago that her sister actually used her to test the mermaid's blood to see if it was safe despite their father telling her numerous times it wasn't. Towa spent decades locked in a cell under her home while Sawa got to get married and have a family, until their father finally died and Sawa let her out, saying "we're all we have anymore".