Thetis acts as the guide to Julian Solo, the hostbody for the God Poseidon, and her role is identical between the manga and the anime, bar the ending. When in the manga, once Poseidon has been defeated; the host is returned to the land and finds a dead rainbowfish nearby on the beach, whence in the anime we see a mermaid swim away after delivering Julian back.
She was defeated by Shaina, but survived this encounter, when she tried to stop Kiki of delivering the Libra cloth and it's weapons to destroy the Pillars of the Oceans.
This means that Thetis is perhaps one of the few saints or equivalents that turned out not to be humans, but an actual mermaid, this only later joined by the Angels of the Heaven Ouverture, even though she is of Danish origin, to tie in with Hans Christain Andersen's fairy tail the Little Mermaid.
The model itself is a Tamashii exclusive, and comes in a very pretty colouring and the tipical 'smuck' face of Thetis is also included, hence I used it on my version.
The cloth is pretty forward to build, BUT the top plastic parts of the underlegs, the 'cups' around the knee area as seen on the picture, are a bitch. They hardly fit and seem to have a will of their own, not really wanting to stay in place once assembled. It basically depends a bit on the positioning of the legs if they keep in their place, but forget putting her in the crosslegged mermaid pose like her object mode.
It is a good model of a bit of a 'cult' character (a bare faced female saint wasn't common pre Yuna) but I doubt she is worth the inflated prices you see around the net lately, I was luckily to get her around the normal 'release' price from around last year at Antwerp Convention, but would have cried if she would have knocked me back the 150+ euros you see her going for today.