DAVID
David was the Animorphs biggest mistake.
He was the one who discovered the blue box after it had been 'lost.' He had just moved to town and was the new kid on the block. Marco spotted him with it a school and told the others. David had put an advertisement on the Internet to sell it and the Yeerks responded by going to his house at the same time as the Animorphs. The Animorphs got David to listen to them and eventually David gave them back the blue box. Sadly, when the Yeerks came they took David's parents and put Yeerks in their heads so David had to hide because the Yeerks knew exactly who he was and they wanted to capture him.
The Animorphs decided they had two choices, 1. To kill David or 2. To make him an Animorph. They chose the second choice and made him the seventh Animorph.
This was also in the middle of their most dangerous mission. The heads of the world were getting together and the Animorphs suspected they were going to make them all Yeerks and they knew one of them was already a Yeerk but they didn't know who.
One night David went to a motel, broke in (morphed as a bird) and just decided to relax, sleep in a normal bed, eat normal food when Jake found him. Jake threatened him saying if he was going to use his powers for things like that the Animorphs couldn't have him around.
Jake, Marco, Tobias, Ax, David, Cassie and Rachel got caught and all but Jake and David managed to escape. David morphed lion (disobeying Jake's orders) and almost attacked Jake (Jake was human) before calming down, He asked Jake who he thought would win if a tiger and lion battled. David thought lion and Jake didn't have a clue but Jake did become wary of David. Jake and David eventually escaped unharmed, only with Jake worrying.
The night after that when they were on a mission, the Animorphs were caught in a trap and David almost went over to the Yeerks but Cassie stopped him. The Animorphs fought Visser 3 and a few of his Controllers before escaping. On the way to Cassie's barn after escaping Jake began to have bigger doubts about David as he went over to Visser 3's side and then came back to the Animorphs. Jake was beginning to think that they may have a traitor on their hands. Jake told the others (through private thought speak) not to tease or doubt David, but to praise him but the whole time as he was cooking up a plan.
Visser 3 had made an offer to David. Go to his old house that night and David would live free with his parents. Jake suspected he would go to his old house that night and he did. Tobias followed David first, with Jake and Ax behind him. When they reached David's house, they found a red tailed hawk on the bed. David said it was Tobias, so after David attacked Jake, Jake sent Ax to fetch Rachel and Jake followed David to the mall roof.
They demorphed and David went lion while Jake went tiger. They fought and Jake ended up on the floor in the middle of the mall. Then Rachel came along and tried to kill David by pushing him into the power lines but David was stronger than Rachel and started pushing her to the power lines when Tobias showed up and rescued Rachel. It turned out Tobias had just got lost and David had killed some innocent bird. Jake was discovered and after Cassie discovered it was him, she managed to wake him and Jake was able to demorph in time.
Next what happened was that David went to the local hospital and acquired Jake and Rachel's dying cousin's DNA and then killed the other Saddler and hide him. Jake and Rachel found him and told him not too and that it was wrong. David ignored them so the Animorphs came up with a plan and told David the wrong one. David also presumed that Tobias was dead so that was an added bonus.
To cut a long story short they trapped him as a rat and left him on some abandoned island, he did return once but after returning for a short time and trying to trap Rachel as a rat he disappeared and the Animorphs never heard of him again, fortunately,
For the full story about David read #20 The Discovery, #21 The Threat, #23 The Solution and for his return read #48 The Return.