From the Super Mario Wiki
Holly Mackerel is a mermaid from The Adventures of Super Mario Bros. 3 episode The Ugly Mermaid. She's the princess of the underwater land of Mertropolis, and the daughter of King Mackerel. During the episode, Mario falls down in the water almost drowning to death. Holly Mackerel saved Mario and took him to Metropolis.
The princess, Luigi and Toad followed them. After she rescued Mario, she fell in love with Mario, mistaking him for a frog prince because Mario was wearing the frog suit where she saved him. Her name is a pun on "Holy Mackerel."
Unlike traditional depictions of mermaids, she has comely female human legs and a fish's head and torso.
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