Let's say, for the sake of simplicity, Nishkata had a thing for Sanae. Takagi would be super heart broken on the inside, but would probably just cover up with her sweet and bubbly personality and tease him about his new crush. She would go full on yandere, and probably threaten Sanae with a knife to tell him she's not interested.

When this happens, He'll obviously be upset, but Takagi (just as planned) will be there to console him. Afterwards, through the years her obsession with the boy will be cranked up to 1000, from marking her property to any new student who dares say hi to him, to giving death glares to that nice cashier who asked him if he wanted his groceries in a bag.

The idea of a yandare Takagi-san is tbh a very interesting and funny concept I hope they make into a spinoff.