Shinji is a narcissistic, sarcastic, yet sympathetic teenage boy.  He is the observational character, sarcastically pointing out everybody else's flaws but his own. Despite this, he is arguably the most flawed person in the show - his crippling abandonment/trust issues dramatically push others away, and his sarcastic attitude doesn't do much to help either. Being as he never had the luxury of having either a motherly or fatherly figure in his life growing up, he believes his only form of survival is to attack their pride, and run.

Shinji walks into NERV with nothing but sarcastic one-liners for everything and anything, not wishing to risk someone approaching him - though, eventually and gradually, he finds that humanity is as terrible as it seems...

Since Shinji is a pubescent teen, his feminine voice is one of his many weak points. His squeaks and squacks define him - much to his chagrin.