Yeah, it's showing 0 if the user doesn't have a name.
Try to find this code in Sources/TPortal.php about line#778
'poster' => $row['realName'],
'poster' => !empty($row['realName']) ? $row['realName'] : $txt['tp-guest'];
That was just off the top of my head, so if it doesn't work go ahead and remove the code. It should say "Guest" instead of 0, assuming that is what you want?