Tassiana and Ronald Ellison
Tassiana and Ronald Ellison are characters that originally appeared in Rachel and Ariane, and by coincidence are on Duquesne at the same time as Rachel and Ariane.
I can solve this mystery. Ronald Ellison is a real estate tycoon who is investing heavily in Duquesne. He is also the one that got Rachel the modeling gig that brought her to the island, and almost certainly arranged for the modeling gig to happen on the island. So not that big of a coincidence.
Tassiana is a multi-talented personal assistant who has no problems using sex to get her way. Ronald is a sex addict and keeps her around for obvious reasons.
There is a “B” plot mystery in the game. After the yacht trip, Ronald Ellison disappears without a trace. I left clues sprinkled throughout the game to solve this mystery, which I will gladly explain, as soon as someone else figures it out first. Any detectives out there?
The mystery was solved by Iago Leonardo Rigo Dalcin on Facebook. Here’s the solution.
Date Ariane Games 
I have an idea. Mr. Ellison is in the Iron Islands, where the inexplicable giant statue of Isis is at.
Mr. Ellison is a devotee of the Roman culture, as explained in Rachel meets Ariane, when he celebrated the Saturnalia, at Elisson Group Building.
My theory is that Mr. Ellison is there to celebrate Navigium Isidis. One of the indications is the statue of Cupid and Psyche present at his private cabin in the yacht. The myth of Cupid and Psyche is part of the Metamorphoses book, which connects the goddess Isis.
As much as Tassiana is a suspect of the local police, I think she is following Mr. Ellison to celebrate the event.
I like your theory, very creative, but not the answer. One of the things you mentioned might mean something in a different context.
Could you give a hint, please?
Someone got it!