I wonder if there is any goddess that resembles a mermaid?
I have looked for mermaid goddesses, but I couldn't find any. I found a legend about Thessalonike, who was Alexander the Great's half-sister. He bathed his sister's hair with water from the fountain of immortality, so she became immortal. When he died, she was so desperate that she threw herself into the sea. The lower part became a fish tail, and the upper part remained human. She would ask the sailors: "Is Alexander alive?" The correct answer was "He's alive and rules the world". In case of a different answer, she would get very angry and start a storm. The real historical character Thessalonike became queen of Macedon and had three sons. When she favoured two of them for the succession over the third one, the latter put her to death.
The only character who looks like a merman in Greek mythology is Triton, one of Poseidon's sons, who had a fish tail (sometimes he is repesentes as having a divided fish tail, as if it were two legs).
The famous "sirens" were, in Greek mythology, women-birds (represented as maidens with wings or else birds with a woman's head). The Greek word for "siren" is the origin of the words used in the Romance languages (Italian, French, Spanish, Portuguese, Romanian) for "mermaid".
One of the siren names that the ancient authors mention is "Ligeia". There is a beautiful short story by the Italian writer Giuseppe Tomasi di Lampedusa, called "Lighea", which can be found in English in "The Siren and Selected Writings". It is about a professor who falls in love with a mermaid. When the summer ends, the mermaid returns to the sea, and the professor spends his life thinking about her.