Emi Takei was phenomenal in the role of Hikari. After her initial psychotic behavior it would seem impossible to root for such a character but she acted with such nuance, balance and heart it was nothing short of amazing. The emotions of being deeply troubled, traumatized, being hopelessly yet intensely in love, the immaturity with a hint of wise beyond her age aspects of Hikari were expressed brilliantly. Compared to the character of Natsumi, Hikari felt layered, complex and emotionally vulnerable in the most compelling way. I was shocked to realize that she had much more chemistry with Shuji and would have made a better couple which would also deliver an unique and fitting ending to the show (after all this show covered all sorts of social taboos).
PS: the student teacher interactions presented in the show were weird and I can bet far from being real, but a forced suspension of disbelief would do the charm-_-