Please, please, stop making Demyx’s Other come from Atlantica. I’m not saying it’s not a cool idea, and a plausible one, but it’s just that everybody and their mom have already done it. There is already merman!Demyx, sailor!Demyx, fish!Demyx, and again, and again...
You’re not restricted by the canon’s limits, here. So be imaginative! Don’t choose the easy way! Surprise your readers (and those who complains “but Demyx comes from Atlantica so your story sucks!” are morons).
And if you really want Demyx to come from an oceanic world, I have good news for you.
Why doesn’t this movie get any love? What has it done to you? Over 22649 KH fanfictions at ff.net, there is only ONE featuring Atlantis.
Control over water, anyone? And, guess what? The world of Atlantis has canonically a heart: a huge blue crystal (named "the Heart of Atlantis" in a burst of creativity), that holds huge power and protects the inhabitants This has so much nothing in common with the movie "Hearts in Atlantis". Here, have another completely random fact, courtesy of Wikipedia: "According to Critias, orichalcum was considered second only to gold in value, and was found and mined in many parts of Atlantis in ancient times." Orichalcum! It's a sign!
Dance, water, dance! Should Demyx be a girl, Kida would totally be her Other. Or else Milo could be, in the name of dorkery. After all, he stayed in Atlantis at the end and power over water comes from the crystals.
Or else (but the timeline is crappy), he could be an inhabitant of Atlantis when it was destroyed. According to legends, the people of Atlantis were more advanced and cultured (hence, a music instrument as a weapon) than the other cultures of that time. They were pacifistic and though they had great power and technology, they shied away from using violence (okay, they ended being corrupted and there was a war and the island had quite a bad fate. But make it an invasion of Hearless and it works!).
Let's stop here before I get even more off-topic. The moral of this post is: there are many worlds out there. Some are over-exploited whereas others are sadly ignored. So don’t be a sheep. Readers (well, at least me) will be so grateful if you explore new possibilities.
On a partially related note, now I would kill for a story where Hades and Poseidon are both working with Maleficient and are complaining about the sheer pain in the ass mythical heroes and Keyblade Wielders are.