Aegon Frey

From A Wiki of Ice and Fire

Aegon Frey, called Jinglebell, is the half-wit son of Stevros Frey and the fool of the Twins. He earned his nickname by wearing a fool's crown of antlers with bells attached.

When Robb Stark came to the Twins to apologize for breaking his marriage vows, Lord Walder Frey had Jinglebell present when he received Robb. Catelyn took interpreted this as an intentionally insulting breach of courtesy. At the Red Wedding, Jinglebell pranced about to the loud music. As the killing began, Catelyn Stark seized him and threatened to kill him if Walder did not release Robb. Walder replied that he would readily trade a half-wit grandson for a son. As Robb was killed, Catelyn cut Jinglebell's throat, killing him.