obligatory adonis fiancée theory: it could be an arranged marriage, maybe even with a member of the Sakumas (i.e. some distant cousin of Rei) but I think it would be sweet and in-character if it was actually a beard engagement and Adonis had agreed to marry a lesbian friend from back home to shield her from judgement. akira has been interested in writing about queer NPCs’ struggles a lot lately (e.g. nobunaga-sensei from akatsuki climax and shu's grandpa) so it wouldn't be a shock!