Ian is really much of a bad person. In the first film at the exposition he was friends with Dave. However in that movie, he took things too far and ended up losing in fortune. In the second movie, he was broke and had the life of a hobo. That is, until the Chipettes showed up. Ian lied to them to get revenge on the Chipmunks, but not on Dave? He might have heard about the accident that happened to him in the beginning and decided not to pursue Dave after all. Ian later takes things too seriously again, and became jobless again. in the third film, he now wants revenge on Dave while he poses as a seagull costume in the film. When they got into an island, Dave convinced Ian that life is more important that untold riches, but he refused. When the gang saw the volcano, he helped the others build a boat to float away from the island before it can explode. Just as Zoe was about to make Dave fall to his death, Ian appeared out of nowhere and told her about the times he wanted to get back at Dave for all those years of riches to rags. Then he helped Alvin pull Dave out of the log with Zoe and made their way to the boat. When they got back to Earth, Ian is once again rich and Dave's ally, and sold Zoe's story to Hollywood. (Although I don't really know what happened to Zoe after the helicopter rescued the main characters.) But on the plane scene, Ian is nowhere to be found. Ian's personality needs to get a hobby.