I wouldn’t say he’s emo…
He’s just a hardened loner type.
His own kind shunned him to the point his own mom threw him off a cliff when he was a newborn just for being born a male….the people who adopted him abandoned him when he was like 5-6 or something.
Hiei is edgy, but not emo. I would define emo as bitter and depressing. In the beginning, Hiei was a homicidal maniac, but he mellows out right after Yusuke defeats him and he's basically one of the good guys for the rest of the series.