A Middleman developer designs and builds static websites using the Middleman framework. They utilize Ruby programming language and Middleman's features such as dynamic pages, layouts, helpers, and frontmatter to create efficient, customizable, and reusable templates. They also manage assets and data, and optimize the site for SEO. Additionally, they ensure the website is responsive and compatible with various browsers and devices. Their role may extend to maintaining and updating the website, fixing bugs, and improving performance. They might also work closely with other team members to integrate APIs and other services into the website.