I have to preface this by saying that you've done a great job of giving Fox's relationship with each of the girls a very natural, organic feel, and this is honestly difficult because I want to see how each one continues to develop as we go along.
1- Maria- Putting aside my thoughts that Maria is adorable, I think she meshes very well with Fox, because her pensive, introspective nature brings out some of those same qualities in Fox, which we rearely get to see since he's always rushing around. I feel like Fox is able to appreciate Maria's personality and style, and coax her out of her shell a bit without being overbearing, and I feel like Maria's quasi-academic, meticulous take on things would be good for Fox in the long term.
2- Karen- Mutual daddy issues may not be he most stable thing on which to build a relationship, but I think both of them being essentially children of two worlds might be. Karen wants to see new things and not feel like she's stuck in one place that never changes, but I think that she'd not actually enjoy the anonymity of the city, while Fox, for all his railing against his workaholic, straight-laced father, brings an urban pragmatism and a desire to do and something to prove that is otherwise lacking in town, and that Karen appreciates. Karen, for her part, is a lot more cynical and realistic than the other girls, which I think appeals to Fox, since it's something like what he's used to and it's less like he's a bandit in a flower bed than with Elli or Popuri, for example. This isn't saying much, since most of the others are very innocent, and Karen still has enough idealism not quite smashed out to make her a great choice.
3- Ann- Combine for ultimate energy! I like Ann, she's got a touch of tsundere tomboy that doesn't detract from her enthusiasm, Both she and Fox have a bit of an impetuous streak, which could work out well/entertainingly
4- Popuri- I really like your Popuri (she reminds me a bit of Aster from your Sims LP) since you've mixed her bubblegum well with a lot of charming idiosyncrasy I think Fox uniquely appreciates her um, individual perspective, and his connection with the land gives them something in common to develop around. I would also be terribly amused by Popuri's interactions with Fox's dad. Still, she might be a little too bright and innocent for things to really work out best.
5-Elli- Elli's interests seem the most narrow of all the girls, and I imagine she'd be sort of a traditional housewife. I'm sure she and Fox could be great friends, but I don't really see a uniquely compelling relationship between them