Heart Eaters float around Soma's head in a somewhat erratic manner and then suddenly dive-bomb on him, feeding from his supply of Hearts. Instead of dropping items or money when killed, they drop several Small Hearts on their trail while dying.
The Heart Eater could potentially be a reference to the Option Eater and Option Hunter from the Gradius series; aside from sharing similar names, they have similar functions, one feeds with Hearts and the others with Options.