It definitely seems implied to be Sekai who saved Sensei:
Even seems to be the one within Kaori considering the nails.
It may also have been what killed the plants after they were perving on Nao-chan. (And perhaps Yumi?:
If that was her.)
Why isn't clear, but it's plausible that she/it simply wants Sensei around. It also seems like this version of him was required to restore it:
There's also how considering the Shapeshifter/Himawari seems to play a part in things related to Sekai, and probably Pareidolia, and how Pareidolia seems to be a fan of Sekai's:
They may all be working together, and wanted to get their plans back on track.
Edit: Based off what Yasu has said, this Sekai thing shouldn't be a thing and is dangerous:
Considering the World Itself is the last of it's kind and stuff, and World translates to Sekai, it may be something beyond the typical supernatural stuff that is taking the form of Sekai.