Is there a way to show the content of a field taken from the user profile in the comment.tpl.php file? How does one display a custom field from a user's profile in comment.tpl.php?
The comment.tpl.php template file receive a $comment value, which contains the comment object. Between its properties, there is $comment-&gt;uid, which can be used to get the user object for the author of the comment, with user_load($comment-&gt;uid). If you are instead interested in the author of the node containing the comment, then you can use the following code.

$node = node_load($comment-&gt;nid);
$uid = $node-&gt;uid;
if ($uid) {
  $user = user_load($uid);
}


In both the cases, the code should be used in the preprocess function, not in the template file used for the comments. In this case, $variables['comment']-&gt;uid, and $variables['comment']-&gt;nid are instead used.