Using "if statements" to add classes in Twig
1) Install and enable the Devel module.

2) Add your /sites/development.services.yml:

# Local development services.
#
# Place this file in the /sites/ directory.
#
# To activate this feature, follow the instructions at the top of the
# 'example.settings.local.php' file, which sits next to this file.
services:
  cache.backend.null:
    class: Drupal\Core\Cache\NullBackendFactory
parameters:
  http.response.debug_cacheability_headers: true
  twig.config:
    # Twig debugging:
    #
    # When debugging is enabled:
    # - The markup of each Twig template is surrounded by HTML comments that
    #   contain theming information, such as template file name suggestions.
    # - Note that this debugging markup will cause automated tests that directly
    #   check rendered HTML to fail. When running automated tests, 'debug'
    #   should be set to FALSE.
    # - The dump() function can be used in Twig templates to output information
    #   about template variables.
    # - Twig templates are automatically recompiled whenever the source code
    #   changes (see auto_reload below).
    #
    # For more information about debugging Twig templates, see
    # https://www.drupal.org/node/1906392.
    #
    # Not recommended in production environments
    # @default false
    debug: true
    # Twig auto-reload:
    #
    # Automatically recompile Twig templates whenever the source code changes.
    # If you don't provide a value for auto_reload, it will be determined
    # based on the value of debug.
    #
    # Not recommended in production environments
    # @default null
    auto_reload: auto_reload
    # Twig cache:
    #
    # By default, Twig templates will be compiled and stored in the filesystem
    # to increase performance. Disabling the Twig cache will recompile the
    # templates from source each time they are used. In most cases the
    # auto_reload setting above should be enabled rather than disabling the
    # Twig cache.
    #
    # Not recommended in production environments
    # @default true
    cache: false


3) Add your /sites/default/settings.local.php:

&lt;?php
/**
 * @file
 * Local development override configuration feature.
 *
 * Place this file in the /sites/default/ directory next to settings.php.
 * This will include the development.services.yml file for twig debug use.
 *
 */
assert_options(ASSERT_ACTIVE, TRUE);
\Drupal\Component\Assertion\Handle::register();

$config['system.logging']['error_level'] = 'verbose';
$config['system.performance']['css']['preprocess'] = FALSE;
$config['system.performance']['js']['preprocess'] = FALSE;

$settings['container_yamls'][] = DRUPAL_ROOT . '/sites/development.services.yml';
$settings['rebuild_access'] = TRUE;
$settings['skip_permissions_hardening'] = TRUE;
$settings['cache']['bins']['render'] = 'cache.backend.null';
$settings['cache']['bins']['dynamic_page_cache'] = 'cache.backend.null';


4) Update your /sites/default/settings.php:

(uncomment this section)

# if (file_exists($app_root . '/' . $site_path . '/settings.local.php')) {
#   include $app_root . '/' . $site_path . '/settings.local.php';
# }


This will allow you to add {{ dump(content.field_name | keys) }} to your template file so you can see just what keys you need to access the correct values.

Try:

{% for key, item in content.field_stat_value %}
  &lt;pre&gt;
    {{ dump(content.field_special_character | keys) }}
  &lt;/pre&gt;
{% endfor %}


Then just add what you need to follow:

{% for key, item in content.field_stat_value %}
  &lt;pre&gt;
    {{ dump(content.field_special_character.mynextkey.0.whatever | keys) }}
  &lt;/pre&gt;
{% endfor %}


I add this answer as being able to print out values will help a designer best. Plus, if your class is not printing, I just think content.field_special_character[key] is not evalutaing to %.

Having debug setup will also allow you to ensure you have the correct naming conventions for your twig template.