I only remember the plot and have completely forgotten the names of both the characters and manga.

This definitely contain spoilers:

The story is starts out like a regular high school manga but the MC finds it strange that he is living the same routine day in day out, then this chick bursts into the room but is ignored by everyone. The guy talks to her and she says that everyone except her is a robot, later on he realizes whats going on and the entire class beats him up. Next day he's forgotten his memories but later regains it after getting smacked by the chick. Cue big robot suddenly chasing them, they get to a big tower, guy tries doing a heroic sacrifice ends up with one arm broken. They then find the class president acting somewhat weird and then they go to this underground boss robot lair, where its revealed the chick is infact a ROBOT!

Found it from Google.

Classmate, Kamimura Yuuka wa Kou Itta.

That's it, right?

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