I have one of two guesses about Rio - she's either exactly what she seems; an annoying, gleefully unemployed freeloader who, while not malicious per say, makes a habit of leeching off her friends and is generally a pain in the neck. Alternatively, she's someone used to putting a happy face on her very real troubles and may suffer either from depression or general "genuinely doesn't know how to adult properly" syndrome. I'm kinda hoping for the latter angle on her character, personally.
Either way, regardless of the reasons for her behavior, I feel like the doom and gloom about her is a bit premature. I do get it - she's set up as an annoying character, so seeing more of her means more exposure to her annoyingness, and she's probably gonna throw Ruriko and Kurumi's home life out of whack, even if unintentionally, for the next while. However, a big theme around her introduction is how it brings up a side of Ruriko that Kurumi hasn't known until now, so I'm hoping that rather than just existing as a roadblock like people fear, she can also bring a new perspective and unconventional developments along with the negative aspects of her presence.
At the very least, I forsee a mixture of her genuinely trying to wingwoman for Ruriko offsetting her disruptiveness, that could be quite amusing if done well. And so far, the author hasn't done anything to hurt my trust in that.