Some themes are designed to display author bylines and some are not. The point of displaying an author name is to differentiate the posts on an multi-author site. Regardless of the theme, and provided the theme is designed to display a byline, bylines will only display now if there are at least two authors who both have at least one published post in the blog. There's a single author workaround here.
Very few themes are designed to have an author profile (gravatar and brief bio) that displays below every post. The data is derived from here > Dashboard > Users > My profile > About You (section) and anything you add there appears on your Gravatar profile page as well.
See author byline and author profile displayed or not > http://wpbtips.wordpress.com/2009/08/15/author-displayed-or-not/
For some tips on Multi-Author WordPress.com Blogs see http://onecoolsitebloggingtips.com/2011/03/06/multi-author-wordpress-com-blogs/