Reccently I write a fic about this ship, so, I did some research and watched the show again, so, this is my theory:
Marlene was going to marry Folken when them was 15, so, for some fanelian tradition, Marlene, Eries and Millerna go to Fanelia for a visit. Eries was 13 at that time, so maybe Eries and Folken were just friends when they meet, and it is possible for Eries and Marlene to fell in love with Folken, because, you know, Marlene was 18 when she meet Allen in a tournament and she was auto isolating for three years, why? maybe that was because of Folken's death. In the other hand, it looks like Eries fell in love with Allen too, maybe Marlene and Eries had similar tastes... well, that's my theory.
By the way, there are really few fanfics about this ship, it's a pity.4/14/2020 #1 Report