Janus

"I'm your best friend, I'm your worst enemy, I'm Janus, God of Doorways. Beginnings. Endings. Choices."

- Janus

Janus is the Roman god of choices, doorways, beginnings, and endings. He is a minor god. It is implied he had joined Kronos, but he later obeys Hera.

The Battle of the Labyrinth
Janus appears to Annabeth and causes her great indecision about the choices she has to make. (Later, she tells Percy that the decision had to do with whether or not to trust Luke), he was also there, thousands of years ago, when the inventor and son of Athena, Daedalus, decided to kill his nephew, Perdix, rather than save him. At the end of the book, Mr. D stated that he had gone over to the Titans' side. He is never heard of again in the books. He makes Annabeth choose one of two doors. When she is having trouble with her decision, Hera interrupts and Annabeth's choice is postponed.

Appearance
Janus has two faces on one head and looks like a doorman to Percy, Annabeth, Grover and Tyson. It is impossible to look at both faces at once, reflecting on him being the god of choices. He almost forces Annabeth to make a deadly decision, but Hera steps in at the last moment and saves her from this dilemma. His left head is generally a pessimist, and the right head is an optimist. Because of this, they often disagree a lot.

Trivia

 * In mythology, Janus is usually depicted as a Roman God of choices, not Greek.
 * Janus has a two-faced head that often bickers.(as a pessimist and an optimist)
 * A moon of Saturn is named after Janus.