Base-Breaking Character: Calder- how sympathetic you find him varies heavily based on how understandable you find his motivations and backstory, especially with his later reveal as a yandere.
Harsher in Hindsight: Victor's early jokes about how obsessed Zerk is with him initially come across as just his Small Name, Big Ego talking- this takes on a much darker tone following his reveal as a 'siren', a specific type of mermaid that inspires obsession in people exposed to them. The same goes for Glen and Vic's first meeting.
Ho Yay: Victor and Zerk, especially in earlier chapters. How much of this is influenced by Victor's nature as a siren is unclear.
Nightmare Fuel: So much in Seaglass, but the crowning jewel has to be Tsukiko's death scene- though not shown on-screen, her face is eaten by a mermaid.
Siren Song
The Woobie: Lotse- seemingly pressured into a marriage he's ambivalent to fearful of, totally unused to the outside world, and prone to fits of anxiety over basic aspects of the reefs outside of Grimmstadt.