[Rio inhales in, then exhales after a pause. She doesn't want to talk about it either, but as long as he's going to go there...]

...It was before that one tournament Ryouga got disqualified from. You challenged me, and I accepted. We ended up in some alley between buildings, where not a lot of light filtered through. Then you played... something, and the whole alley caught fire. The rest is a blur, but... I do remember someone carrying me out.

[By process of elimination, she's already figured out it had to be him, but she doesn't say that just yet. She grimaces, staring him down in a way that indicates she's wondering why he's jumping straight into the trauma talk. But perhaps that's part of his "fanservice," too...]

Is that enough detail for you?