Onomatapoeia is DC villain and an enemy of both the Green Arrow and Batman.
Onomatopoeia's name and origin is unknown. He is a serial killer whose M.O. is to murder non-superpowered vigilantes. He mainly goes after the Green Arrow, but he's also a major villain of the Batman miniseries Cacaphony.
Onomatopoeia is infamous for gaining Batman's trust (disguised as a vigilante named Baphomet) to such a degree the Dark Knight took him to the Batcave, shared geniune emotion with him and introduced him to a friend - Batman was even about to reveal his true identity to the disguised serial-killer when Onomatopoeia revealed his true self and murdered Batman's friend in front of his eyes.