So I’ve been doing some research and I think the top three templates are ...

A Beast which has achieved its Inheritance by becoming a Beast Incanate, this would allow for transformation and through Heroes and the like an element of High Fantasy could be explored possibly with Changeling involved. Beast Incarnate are extremely power needing a minimum of Lair 8 and literally forming their own Myth.

Vampire is another obvious possibility with Auspex and Kindred senses giving Gemini a way to see while still being blind and a strong manifestation of Protean explaining the ability of transformation.

Demon also presentes a possibly with Demonic Forms and Glitches explaining some of the markings. They may not be full fledged Demons though they could be fallen angels, this is just something that would be different.

They also do not need to be the same template per say we could do some combination of the above.

For Thanatos I was thinking of having him be the center of a Krew of Sin Eaters mythology which will work well with either Beast or Vampire

Maybe Beast for him and vampire for her?

We could do something like that what Clan would you pick for Gemini one of the regular ones or one of the other Clans ?

I’d need to look at the options before picking.

Fair enough