We are...the watchers...on the Wall.
Who: Qhorin Halfhand
Source: Episode 2x10, "Valar Morghulis"
Note: Having been captured by the wildlings, character provokes a fight with fellow prisoner Jon Snow in order to allow Jon to gain the wildlings' trust and infiltrate their army by killing him. The line, spoken after being mortally wounded, is taken from the Night's Watch vows, reminding Jon of his duty.