A term once used to refer to a certain style of music in the late 90s, then to a teenage stereotype in the early 2000s, nowadays used to make insecure people defensive (hence the many definitions trying to dissuade people from its common usage). For the teenage stereotype, "emo" connotates angst and nihilism; a fetishization of life's unfairness or meaninglessness; Reddit atheism without a god to denounce. This may be accompanied by attitudes and appearances reflecting a teenager's expected capacity for such extreme ideas.
Hamlet and Shadow the Hedgehog have one thing in common; they're both emo kids.