Technically, Mara isn't a mermaid. She's a former Cobra Eel that was experimented on by Cobra scientists (probably an early attempt at what would later be perfected with the Hydro-Vipers.)
But Shipwreck calls her a mermaid, so why not?
Mara was a good one though. She had a good enough back story that they brought her back later (sort of) for "There's No Place Like Springfield." She was also smarter than most Cobra soldiers.
For Mara, I used Heroclix Colleen Wing. Her skintight costume works well as a Cobra wetsuit and her long hair fits Mara. I trimmed her katana down to be more like a knife. Mara with a katana seemed a little too much.
Also, I tried to make he skin a little purplish like the cartoon.
She kind of looks like an X-men character now.