While Sarah isn't allowed to answer everything, she did tease a bit about Roxy, saying to a person they'd have to wait and see. With a winking text emoticon, to boot. So while that's not exactly a yes, it would seem Roxy is at least planned to be in this game, and likely playable at some point. But how far might they have gone towards implementing her? Recently, I've dug up some media and info that suggests she might well be there, and in the game as of release. (Whenever that may be.)
Let's turn to that silhouette at the head of this article; it's from a post made three years ago on RCRU's Facebook page. It is quite obviously a female, though it doesn't look much like the official art of Roxy, who in the Japanese original was named Hasebe. Yet whoever she is, we might just have gotten a fuller view of her since then, courtesy of this shot:
|If you squint, you can see a very similar hair style blending into the carpet.|
Scott Pilgrim Vs The World, referential to video games in general, but specifically modeled on RCR, also has a character called Roxy, and this is what she looks like. The similarities to the girl we've been looking at so far in this article, though not absolute, may well be pointing in that direction. Since Conatus Creative is, much like Scott Pilgrim, Canadian and in love with River City Ransom, it makes sense that they'd derive at least some inspiration. (Update: Since I wrote this, I saw a video interviewing Bannon Rudis, confirming that yes, a Scott Pilgrim Alumnus contributed to this game.)
All this is to be treated as speculation rather than a definitive answer, but I think it's based on some rather convincing evidence, so stay tuned!
Update: Another image with this character has been located at Kamgraphica, a web design company who took on RCRU as a client. She is in the same pose, which likely implies it's her standard fighter stance.