The Sine Corda are those that have been "touched" by the Quiet. They claim that due to the Quiet, there is a hole where their heart should be, hence the name which is Latin for "without hearts."
The first Sine Corda recorded was Number Nine. She claimed to be an "Agent of the Quiet and a Member of the House of Nothing" and that there "was a hole where her heart should be." It is unknown if this was true or merely a delusion she had.
The second Sine Corda seen was Than Roex, a former operative for the Eden Facilities. He was touched by the Quiet during an incident where the White Room in Facility Eight was breached and claimed that he, too, had no heart.
The third Sine Corda was responsible for the kidnapping of the Free Radical, though only for the purpose of disseminating information and almost immediately returned him back to his home.