>Do we know what strictures apply here? In terms of our right to take ownership or whatever?
>You are correct in your thought that you wouldn't even need to formally challenge the Knowledges for ownership of Reisen. All you'd need to do, in the simplest terms, is beat them up and take her papers. It is rather crude, but that's how the Oni operate. For Byakuren Hijiri to own two slaves at once would require you to spend more time at work, but apart from that, there's no 'legal' reason why you couldn't own both Rin and Reisen, not from an Oni perspective.
>There would be another complication you can think of as well. The maximum occupancy for the Moonside apartments is two. It's usually only 1, but Reimu made an exception for Rin and Reisen, given their circumstances. You wouldn't be able to keep them both in your apartment.