Sider for iOS is the best ChatGPT iOS app. It offers free chat with hundreds of robot friends. You can also customize robot prompts to create robot friends who understand you best. Besides ChatGPT, it also supports other professional robots, including Bard and Bing, and is constantly updating. You can also turn PDF files into robots to help you understand text content faster and better. Sider for iOS supports multiple API settings. You can use your own API key without any extra fees.