Ammon Jerro (also known as a Warlock) was once a court wizard with a title of a jester, but when he found out about the Illefarn guardian, what it had become, and what it was going to do to all of Abeir-Toril, he changed. He cut all ties with his family and friends and immersed himself in years of study and training in order to better face the Illefarn construct gone awry. In completely losing himself to zealous training and searching for powerful allies, items, and henchmen, mainly of the demonic sort, he loses bits and pieces of his humanity and soul, until he becomes a single-minded weapon favouring isolation, ruthlesness and efficiency. Ammon Jerro will do what he thinks needs be done in the most expedient way possible, without regard to loss of life or other (in his mind) miscelania. He is there to destroy the King of Shadows, everything else fades. He is present in the NWN2 campaign from the start, being the human character that faces the shadowy opponent in the intro of the game. He has a prominent glowing tattoo on his forehead, which is often remarked upon by Bishop who calls him 'tattoo-face'. He was also the only one who faced the Illefarn guardian alone, and survived.
He is the grandfather of Shandra Jerro, whom he slays not knowing who she was in an act of blind anger, thus ending his bloodline. This breaks him, as Zhaeve points out, and Ammon, for the first time in who knows how long sees himself for what he has become, and as a result mellows a bit, surrendering his allegiance to the PC, but he doesn't give up, not until he faces the King of Shadows one last time. A strong and a tragic figure.
Ammon Jerro is also present in the Mask of the Betrayer campaign, where the PC can choose to buy his displaced soul from a pair of merchant devils, acquiring his skills once more and delaying Ammon's inevitable eternal suffering, once he is finally dead, at the hands of all the demon and devil lords he once imprisoned under his service. If revived, he will aid the PC in Kelemvor's city, paying off his debt.
Source: Wikipedia, "List of Neverwinter Nights characters", available under the CC-BY-SA License.