Nina

obsessed with the student council president

has no idea that it's mutual (initially)

passes as a normal chatty scatterbrained airhead

not a genius, but can provide some down-to-earth wisdom in a pinch

routinely threatens people close to her object of interest with typical school bullying like thrashed lockers and torn textbooks

no death threats though, she doesn't think that far

constantly scratches her cheeks, hence the band-aids

the concept for her and her future boyfriend was "two ineffectual yanderes" so the most harm they cause is to themselves; everyone else is mostly freaked out (and suffers property damage)