Parker was in the Pillar of Autumn with the Master Chief when the Covenant raided the ship. Like all the cowards aboard the ship, he abandoned the ship and helljumped to Halo with a Human Entry Vehicle. He, during the rape and pillaging of Lies and Retardation with Chief, later got wounded when humping a Hunter's back too hard. He cried a lot but didn't let the pain stop him, he cut all his nerves so he won't feel a thing so he could still help rescue Keyes.
He let the Chief go without him and his ODSTs. After the Chief rescued Keyes, the ODST bodies were found around the floor with two Elites with Active Camouflage. Parker was found dead with his suit tear-soaked. Simply put, he cried too much.
Later, his wife confessed that she had worked at hooters and had never actually had an orgasm. Isn't that horrible?