David was a fairly average kid who had the misfortune of finding the Escafil Device. When he tried to sell it on the Internet, Visser Three showed up at his house to claim it. The Animorphs intervened, and escaped with the device and David, but left his parents to be infested.
Realizing David had seen too much, the Animorphs decided to let him join their ranks. However, David could not adjust to the new lifestyle, and rebelled by using morphing to commit crimes in an attempt to simulate his normal environment.
When they were cornered by Visser Three on a mission, he gave away who he was and attempted to negotiate with the Visser. He managed to recover ground and attack, later rationalizing it as a bluff he had planned all along.
Eventually, David rebelled, and tried to kill the Animorphs using his new power. The Animorphs then trapped him as a white rat to prevent him from allying with Visser Three or the Yeerk Empire. They took him to a large rock and deserted him there. Apparently, a boy claimed the rock was haunted. He said when his family went on vacation, he heard someone wailing "No! No!" as they passed by, which was David's thought-speak.
He appears later in The Return.