I'm hardly her biggest fan, but I think Riona has a lot going for her. To start with the obvious, she covers the goth and asian niches for the game, and she has a pretty distinctive look. She also gets to be the voice of reason in Quinn's cut scenes, pointing out the obvious flaws with her boss' plans - always a good way to build up audience sympathy.
On top of that she has a fun, uncomplicated personality: she likes music in offbeat ways, jokes when she's in a good mood, and generally tries to enjoy life. She also enjoys sex, and seems openly fond of the MC in a way Mel or Sarah do not (and Quinn refuses to admit). And now that we know she "can't" come from sex, she's an obvious challenge waiting to be bested.
Lastly, while Riona is generally quite independent there is just a hint of damsel in distress about her (we're talking maybe a deciMaya here). As the black hole around Quinn gets larger and larger, we can see Riona is going to get caught up in it. She even bares her soul to the MC about these problems on occasion. But Riona seems unable to take that final step and break free on her own (driven home in Episode when Quinn says she's only good for life in the restaurant). It gives the sense that maybe the MC can toss Riona a lifeline and help get her life back on track.
And who wouldn't want that?