I was following this interesting topic silently and now I decided to put some words on it.
First, I don't think it is possible for Sari to be Rhys' daughter. Considering that Lena was a princess, she wouldn't want to have a baby from a guy she didn't marry. I know the fact that she was Rhys fiancée makes the connection possible, but I don't think she would do that, considering the responsibility she has as a princess. From the game we can get that she is a responsible person and she sacrifices her position as a former Rhys' fiancée to help him to find Maia, the girl he wanted to marry.
By the way, I think Lena being Rhys' promised fiancée not a good idea for the game, since it raise lots of questions, some which I addressed in a fanfic. If Rhys was promised to a neighbor kingdom princess, why did they let him marry with a complete stranger? We can consider that Lena helps Rhys because she really loves Rhys and wants him to be happy, regardless if it is with her or not. We can consider this true if Rhys really loves Maia and is not only infatuated with her. But would Rhys really love a girl he knew months before and with amnesia? Would Lena really love Rhys when it is clear in the game the they have never met before the first encounter on Landen palace dungeon?
Considering these questions, I can only come with the conclusion that Rhys is just infatuated with Maia, who is said in the game to be the cutest girl and sweet. Lena was just infatuated with Rhys. Rhys, after the quest, could really fall for Lena, since she sacrificed herself for him, but I don't think Lena would love Rhys in the end of the quest, seeing that he did everything he did because of another woman. Maia too would, most probably, not love Rhys because she would have the time to do what Rhys' father suggested and he didn't: to cool of and see her responsibility as a princess. In doing so, she would realize that Rhys was just infatuated with her, forgetting his responsibility as a prince, and that she would only marry him because she didn't have a better alternative. Rhys would end without both Lena and Maia and would settle by marrying Mieu, who was made to serve his master, so the perfect wife
(just kidding, girls
). But I really think Rhys doesn't deserve neither Lena, who he jilted before, nor Maia, who he was abusing with his power.
A person only sees and hears what he wants to see and hear and disregards the rest.