Kevin Smith discussed the character in a 2007 interview:
When I did Green Arrow, I went with Onomatopoeia for a villain, just because I loved that word, and it kind of formed the character inasmuch as he would say sounds out loud. It only kind of works – I think – on a comic book page because if you have a gun going off, they usually write BLAM! and then you can have, you know, the character saying “BLAM!” in a word balloon, but like if you tried to do that cinematically you can’t really rock it. A gun in a film sounds completely different. It doesn’t read as BLAM! and so to have a dude say BLAM! after a true gunshot, all these people would be like ‘he’s just retarded’. I think it works great in print and on a comic book page. I don’t think that character would translate very well outside of that.
Onomatopoeia is a serial killer who targets non-powered, vigilante superheroes. He earned his name because he imitates noises around him, such as dripping taps, gunshots etc. It seems that he may be able to perfectly imitate these sounds but that has been left ambiguous. He once stated his nickname to one of his victims after he shot and killed her. This was the only time he has been seen speaking normally. He invariably carries two semiautomatic handguns, a sniper rifle (which may or may not carry live ammo along with tranquilizer darts), an army knife, and is a skilled unarmed combatant. His usual outfit consists of black gloves, pants, and shirt, with a long black trench coat and a full, black hood decorated by concentric white circles.
One of his notable targets was Connor Hawke, the second Green Arrow. He shoots Connor, who is saved by his father Oliver Queen (the first and original Green Arrow). While Connor is being operated on in the hospital, Onomatopoeia returns to finish the job. He kills several doctors in the operating room but is stopped by Queen and Black Canary. Queen holds the villain at bay with an arrow ready to fire at extreme close range. The stand off lasts for sometime, until the surviving doctor stabilizes Connor. Nevertheless, Onomatopoeia manages to escape.
He is later recruited by the Secret Society of Supervillains, as part of an army that is sent to conquer the city of Metropolis in the "Infinite Crisis" limited series. A superhero army, backed up by the National Guard, successfully opposes the Society. Onomatopoeia is seen being punched in the face by the 'Odd Man', a costumed, un-powered vigilante, exactly the type the villain has slain before.
It remains uncertain whether or not Onomatopoeia is a metahuman, however in his encounter with Green Arrow, he took six arrows without stopping. He took two to one shoulder, one to the other, one in between the first and middle knuckles of his right hand, one through his right foot, one through the palm of his right hand, none of which slowed him or even impaired his manual dexterity. After being shot, he took a seven story drop off the roof of the Star City hospital and still managed to run away before Green Arrow could find him.