Luis Castillo apparently already is happy enough with the Mets to have interest in staying with them next season. Castillo, acquired Monday in a trade with the Twins, joined his new club yesterday and was in the starting lineup, playing second base and batting second against the Brewers. He went 1-for-5 in the Mets' 4-2 loss. Before he played his first game with the Mets, Castillo, who can be a free agent after the season, said he'd like to return to the team in 2008. "I hope to stay here," he said. "I want to stay here."