The growth of ubiquitous computing and Web services is expected to make everyday life of people more convenient, and it is also expected to make the society safer and more active. However, the problems of the Internet age such as the digital divide, network criminals and the insecurity of privacy prevent many people from seeing the benefits of the Internet. To solve the problems, we have been studying a methodology which aims at developing a cognitive agent which supports the social activities of a person based on a symbiotic relationship between the person and the agent. The symbiotic relationship means that the person and the agent are tightly coupled in a ubiquitous environment and keep their partnership to support each other, according to Licklider’s symbiosis. In this chapter, we propose a concept of a symbiotic agent and an agent model to design and implement functions defined in the symbiotic agent. To realize the concept, we have implemented an agent platform to run multi-agent systems consisting of primitive agents using Steel Bank Common Lisp running on Linux and MacOS.