The poetry being by the checks, makes me think that Spirit Sekai is the one keeping track of something. It fits her "Teacher" persona as well.
Not to mention, someone/something scribbled out Maya's note by the checks in "The life of Prizes" event, and seemed to hide the checks with a big X in Maya's birthday event, and it's clear that Sekai hates her.
You also can hear a female voice in "The life of Prizes" saying she loves Sensei and wants him to tell her he sees her, which seems to have been Spirit Sekai this whole time. Especially after how she tried to get Sensei to see her in the japanese event that restored her, which also had a female voice mentioning the first time they held hands. Spirit Sekai may also have been the voice that was in "There was Nothing" that was trying to keep Sensei to remain calm and told him the port number, etc.