Though at one point Robin asks what Amon's ability is, he never answers and is never revealed. Originally, he is reluctant to team up with Robin, as he is told to "keep an eye on her" by STN-J (Japanese branch) boss Zaizen, as Robin is sent from Solomon (headquarter). However, seeing her worthiness in cases concerning highly prominent witches, he eventually accepts her. Later on in the anime, Amon disrupts Zaizen's plan to eliminate or take Robin into Factory. Soon after, Solomon attacks STN-J, and once again Amon saves her.
After that episode, Amon is not seen in STN-J anymore, but found to be working directly for Solomon. He is ordered by Solomon to hunt Robin, as she has learned the arcana of the craft and her powers have grown beyond Solomon's control. However, when meeting face to face with Amon, Robin fully accepts Amon's decision whether to kill her or not. Amon finds that he cannot kill her.
It is revealed near the end of the series that Amon's father was a witch and his mother a "seed", indicating that Amon himself also carries the dormant witch trait. Amon fears the awakening of those dormant genes. Amon's mother willingly accepted his father's lead and she "awoke" as a witch and from then on, she no longer recognized young Amon. His fear and hatred for witches originated from that moment, though his behavior throughout the whole 26 episodes is neither particularly hateful nor fearful toward witches. At the last episode, when Robin realizes that her role was to make witches and humans live together in peace as "Witch Eve", Amon volunteers to be a watchman who would terminate her if she turns destructive.
Amon and Robin's survival is not known, but Karasuma was with two of them until the last moment, and she emerges alone. Karasuma's unconcerned answer "don't know" suggests that Robin and Amon escaped safely. The relationship between Amon and Robin has not been any more than a professional relationship. However, Amon breaks up with Touko, Zaizen's daughter. They seemed to be already falling apart somewhat. Touko calmly accepts that and suggests that Amon "changed," suggesting that Amon has become a warmer person to say such things as "suddenly I thought of you and called." While Amon and Robin are never seen in romantic light, Amon repeatedly goes extra lengths to save Robin, and Robin would trust her life with Amon.
