Great news, folks!
Rivalhearts is verified playable on the Steam Deck!
A general playthrough should be seamless, though there are just a few potential moments you may have to circumvent. We'll go over them below:
Selecting Who to Pursue
The main issue we encountered overall was this screen being janky to navigate with only the thumb sticks. It would be best to use the touch screen at that moment.
Keyboard Finickiness
There is also inconveniences when it comes to using the keyboard, but this is only relevant currently for renaming save pages. The keyboard will go away after first showing, so to bring it back just press Steam + X. As well, the keyboard may show up on top of where the text is being edited; this is fixable by just clicking the three bars button, which shifts the keyboard to the opposite side of the screen.
That's about all we found that could cause a notable inconvenience when playing.
Thanks for reading!