Responses

They're the couple from the original(?) series, I believe it's Hitorijime Theory.
Since its a spin-off, artists/editors/publishing houses generally include a oneshot of the original couple/s as a throwback and to create interest for new readers...its just good marketing and it plays to the loyal readers of these artists too, to be able to see a beloved couple after their original ending.

happy to help! hope you love them too! ٩(๑❛ᴗ❛๑)۶
This last update was not needed, better of as a one shot since it didn't make any sense.