Alecto

Alecto is one of the three Furies.

The Lightning Thief
In The Lightning Thief, Alecto is disguised as Mrs. Dodds, a pre-algebra teacher at Yancy Academy. She disliked Percy Jackson and it was later discovered that she was actually one of The Furies and not really human at all. On a field trip, Percy kills her with Riptide and she disintegrates into dust. He later fights her again while he is on his quest.

The Last Olympian
Mrs. Dodds appears again in the final book. Her real name was revealed to be Alecto, and that she works for Hades. She was the "lawyer" who took Nico di Angelo and his sister out of Lotus Hotel.