I found out another mistake on line 82 of charlist.php
In case you don't select any enemy related to a superhero, the following code returns an undefined index error, because it looks for $enemies[$row['id']] that is not set:

if (!isset($enemies) || ($enemies[$row['id']]=='')) {
      $ene = 'none';
    } else {
      $ene = $enemies[$row['id']];
    }
}
    $table .= "<tr bgcolor=\"#" . $bg . "\">" .
              "<td><a href=\"charedit.php?c=" . $row['id'] . "\">" .
              $row['alias']. "</a></td><td>" .
              $row['name'] . "</td><td align=\"center\">" .
              $row['align'] . "</td><td>" . $pow . "</td>" .
              "<td align=\"center\">" . $ene . "</td></tr>";



So I changed it as follows and it works!

if(!isset($enemies[$row['id']]) || ($enemies[$row['id']] =='')) {
    $ene='none';
    } else{
    $ene = $enemies[$row['id']];
      }
    ...

:D