These are tough costumes. I myself am mid-way in creating a Leelin costume! ^_^
What I'm doing is creating a skin-tight off-white/silver-ish bodysuit that is like a swimsuit with gloves attached. It also has attached fins in the back and in the arms.
For the legs, they almost look like pants over the top. So that's how I'm making them. Loose in the hips, and out of a strong fabric that will slightly hold its shape. (I plan to add foam padding to shorts under them once the pants are in good shape.) Then I've sewn layers of organza "scales" over the top in multiple colors. It looks a little different than the art that way, but it still looks fantastic.
So they get tight around the ankles, I'm planning to zipper the botttom of the legs, behind the fins. There's also a zipper in the back of the pants, to get them on.
I'm nowhere near finished with this costume, and I'm trying to finish a billion other things right now too... (including moving to Japan...) so I've no idea when this costume will be done. But I've started a lot of it, so if you have questions, feel free to ask! I should be able to offer at least a little help!
Good luck! Hooray for obscure Suikoden costumes!!! I'm so glad someone else is making one of the mermaids!