From the japanese wiki:
http://image02.seesaawiki.jp/f/a/fantasica/60dcb661941882e3.jpg
http://image01.seesaawiki.jp/f/a/fantasica/04098e16a64bb4b6.jpg
http://image01.seesaawiki.jp/f/a/fantasica/b351ca5e05dfb04f.jpg
She's a mermaid getting attacked by an octopus, and looks like she's the princess of the mermaids or something like that. Her sprite looks adorable. I have no idea what's going on with it after the third frame though.
It appears King Arthur is a 7* monster. most likely recruitable afterwards.
I love mermaids.
Dammit.