What variable will let me access user profile info?
If this link is to be on the user profile page then you can grab the related user ID from the internal URL (using the arg() function), and then use user_load() to load a user object that contains the username:

// Get user ID from internal router path (this isn't necessarily the URL shown in the address bar, it's the internal path that Drupal is using to serve the page).
$uid = arg(1);

// Load the user account and extract the username
$account = user_load($uid);
$username = $account-&gt;name;