Its imposibble to having they back (For execption for Rolf)they will leave your town forever, in Animal Crossing: Wild World, if あなた befriend a villager, they will give あなた picture of themselves so あなた won't forget they forever, the picture are rare and can't be found anywhere else. NOte: Rolf will 移動する to your town if あなた talk to many times if あなた visit your friend town with Wiiconnect24/DS wierless communication または WFC, if he wants leave your friend town, talk to him in many times, he will sometimes 移動する back to your town, hope I helped :)
