Who held Percy's mom hostage in Percy Jackson and the Olympians: The Lightning Thief?

In the story, it is Hades who holds Percy's mother (Sally Jackson) hostage.
Earlier in the story, Zeus's master bolt is missing, and everyone suspects that Hades has it. However, Zeus believes that Percy stole the bolt (on behalf of his father, Poseidon). Athena, the goddess of war, is supporting Zeus in the conflict. Meanwhile, Apollo, Aphrodite, and Ares are firmly in Poseidon's camp.
It is Ares who reveals that Hades is keeping Percy's mother in the Underworld. For his part, Hades believes that Percy stole both Zeus's master bolt and his own Helm of Darkness. The god of the Underworld thinks that Poseidon has used Percy to steal his most powerful weapon. So, Hades kidnapped Percy's mother in order to lure Percy to the Underworld.
Unbeknownst to Percy, the master bolt is in his backpack, planted there by Ares himself. For his part, Hades thinks that Percy will use Zeus's master bolt as a bargaining chip.
Knowing that he does not have the helm of darkness, Percy has to leave the Underworld. Back on earth, he confronts Ares and engages him in battle. Eventually, Ares gives up the helm of darkness, and Percy brings it back to the Underworld. Later, Percy learns that his mother is alive and well.