The big takeaway of course is confirmation there was a 5th member of De:Lphinus. There's been subtle hints beyond Masaki plain asking where the 5th girl went, Delphinus being a 5 star constellation for example. And the Sailor Moon theory (each member follows the same naming pattern as the 5 main sailor senshi: Tsuki/moon, Ai/love, Hi/fire, Ki/tree, Mizu/water is missing. Dorothy being a massive Sailor Moon fangirl and referencing this way back with Kidou Ange wearing a Jupiter shirt.) So 5-chan is almost certainly Mizu-something.