I hate that he was introuced with subtlety of a sledgehammer. The manga has created tons of opportunities to move the plot along and then wasted them all, and now we're getting this psychotic little shit appearing out of nowhere to brute-force the progress?
This was pretty obviously forced onto the author by the editors, see the whole drama with the 1yr hiatus.
Personally, I always assumed that the author planned to integrate Jun and Shizuku into the realization, and end it right after the school festival, but it was too popular on MagXIV to let end. Then, author gets burnt out and does self contained stories, and eventually is forced into something they didn't want to do.
Anyways, thanks for having that last page! Honestly, I do think the author lowkey forgot about Soumei meeting Hiyoko+Hana, although they might be using it as a "Get of out Jail Free" card for why Shuumei thought they were siblings.