There are five mermaids in total. They all look similar to each other, apart from their hair and tail. There is a red-haired mermaid with a teal colored tail, a blue-haired mermaid with an aqua colored tail, a black-haired mermaid with a green colored tail, a purple-haired mermaid with a lime-green colored tail, and lastly a blonde-haired mermaid with a lighter green tail.
The mermaids are first seen stealing in Bikini Bottom from their singing. Their reason for stealing the objects is so they can find something "cool" to wear. When they sing their song backwards, it returns the enormous pile of the stolen objects back to Bikini Bottom. They then decide to go to the mall with Pearl because they want to find out what a mall is. Patrick is then revealed to have a crush on the mermaids. Unlike Mindy, the mermaids look confused seeing Patrick when he wants to hang out with them.
"Mermaid Man Begins" has Mermaid Man's version of his backstory including the lead mermaid. She saves him from drowning and gives him a magic sea star that allows him to breathe underwater. The blue-haired mermaid also helps. They have more realistic appearances in this episode, but exactly the same colors.
Like Mindy, Patrick has a crush on them because he loves mermaids.