Mermay's over, but I'm not done with it! Anyway mermaid Tang Xuan Au where Tang Xuan is a octopus based mermaid and Li Ling is like a rich aristocrat. They meet when they are kids and become best friends then Tang Xuan goes away. So a few years down Li Ling is an adult and thinks he made it up till Tang Xuan shows up again. Lewis is also going to be in this idea (cause he deserves it) but I don't have an exact idea. I'm thinking sailor who was friends with Xuan and Ling when younger and also has to go away but stayed in contact with Tang Xuan

You put down Octupus!Merman!Tang Xuan, solid as octopus are know for their flexibility and adaptability, but I raise you WingedFish!merman!Tang Xuan, specifically Four Winged Flying Fish. Fast, sleek, [small,] and can perform decent feats of acrobatics when it launches itself out if the water and into the air. They use their ability to speed boost to outmanoeuvre predators, so they're agile too!

Lewis would be a legend, spread in multiple small towns and isolated villages, as a pirate who shows no mercy to other ships that cross his territory, his wrath setting ships ablaze with a terrifying plume of fire that the townsfolk can see clearly from the comforts of their beach. The screams of these tortured sailors travel with the wind from their emblazed ship, providing easy fodder to scare unruly children into doing their chores and listening to their elders.

In reality, Lewis is in a petty sibling cat-and-mousrle game with Bonnie, Lewis actually being part of the Western King's admiral fleet and trying to catch up to his Pirate!Sister before the Royal Navals find her. She smuggles herself onto pirate ships, in the hopes of joining a crew and going on bloody adventures, but since she's a bit too... Excitable... And aggressive... And bloodthirsty... The Royal Naval fleet always gets involved, and Lewis is almost always there to at least rangle her off the ship. He is fine with his sister's dreams, he just wishes they were a tad less violent so he didn't have to lose sleep over her potentially getting caught and sent to the gallows.

With that being said, Bonnie is a bit of a legend herself, among the pirates, as her figure holding a crude, hodge podge farming scythe in one hand and a ball of fire (actually a lit flask) in the other, is the omen before their ship explodes and they're left to die a slow death. She appears out of nowhere and vanishes just as abruptly too.

Then everything you said is happening between our two boys, with the added spice that Li Ling goes off to find his friend, with the idea to find this Legend and join his crew, so that there will be no rules stopping them and no eyes and ears reporting back to his family. Of course, he hops on his Best Friend's™ ship, Lewis, not realising that this man is the Legend and Lewis can only keep a poker face and act like this Fire Fist legend is someone he's met rather than himself.