TWiki makes names available in three formats: USERNAME like jsmith, WIKINAME like JohnSmith and WIKIUSERNAME like Main.JohnSmith. Un-authenticated users are all TWikiGuest.
Syntax: %USERNAME%
Expands to: guest
Note: When used in a template topic, this variable will be expanded when the template is used to create a new topic. See TWikiTemplates#TemplateTopicsVars for details.