Origins

She is centuries old, and has a great mastery of magic. She lived in a home on Whisper Hill, until it was destroyed (Fantastic Four 1:134). She was a nanny for Franklin Richards and was the first to warn the parents of his reality power. She also helped the Scarlet Witch grow in her powers. She has died twice. The first time, she was burned at the stake by her own family. The second, was by the hands of her student, the Scarlet Witch.

Powers

She is a witch, and has a great mastery of the mystic arts. She can also create illusions, move objects, project forcefields and thoughts and teleport. Agatha Harkness' mystic abilities are so vast, that she is 1/3 the earth's High Priestess, a title that she shares equally with her former student, Scarlet Witch, and former X-man, Storm. Her title of High Priestess, and a slight hint from Wanda, suggests that Agatha may still be alive.

Her pet cat, Ebony, can mutate into a mystical panther and is very dangerous.

Ultimate Agatha

Oh yeah, she does have I.D.

In the Ultimate Universe, Agatha is a Colonel in SHIELD with Alpha Clearance. She visits the Baxter Building to have words with Dr. Franklin Storm. She feels that the needs of the Fantastic Four are not being met. She says the history of the program they are running is one of squandered potential and wasted opportunity. She proceeds to interview each member, asking questions about their personal lives and relationships. She seems to want to really connect with Johnny Storm.